Emissive ceramic materials having a dopant concentration gradient and methods of making and using the same

ABSTRACT

Disclosed herein are emissive ceramic materials having a dopant concentration gradient along a thickness of a yttrium aluminum garnet (YAG) region. The dopant concentration gradient may include a maximum dopant concentration, a half-maximum dopant concentration, and a slope at or near the half-maximum dopant concentration. The emissive ceramics may, in some embodiments, exhibit high internal quantum efficiencies (IQE). The emissive ceramics may, in some embodiments, include porous regions. Also disclosed herein are methods of make the emissive ceramic by sintering an assembly having doped and non-doped layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Application No.61/418,725, filed Dec. 1, 2010, which is hereby incorporated byreference in its entirety.

BACKGROUND

1. Field

The present application relates to emissive ceramic materials having adopant concentration.

2. Description of the Related Art

Solid state light emitting devices such as light emitting diode (LED),organic light emitting diode (OLED) or sometimes called organicelectroluminescent device (OEL), and inorganic electroluminescent device(IEL) have been widely utilized for various applications such as flatpanel display, indicator for various instrument, signboard, andornamental illumination, etc. As the emission efficiency of these lightemitting devices continues to improve, applications that require muchhigher luminance intensity, such as automobile headlights and generallighting, may soon become feasible. For these applications, white-LED isone of the promising candidates and have attracted much attention.

Internal quantum efficiency (IQE) is the ratio of photons created by anemissive material to the photons absorbed by the same material. Anincreased IQE value can improve a lighting apparatus's energyefficiency; however, there is still no reliable method for increasingIQE without also diminishing the luminance efficiency. Thus, there is aneed for new emissive materials that can overcome these limitationsregarding IQE.

SUMMARY

Some embodiments disclosed herein are emissive ceramic materials havinga dopant concentration gradient along a thickness of a yttrium aluminumgarnet (YAG) region.

Some embodiments disclosed herein are emissive ceramic materials havingone or more porous regions.

Some embodiments disclosed herein include an emissive ceramic comprisinga yttrium aluminum garnet (YAG) region and a dopant having aconcentration gradient along a thickness of the YAG region between afirst surface and a second surface, wherein said concentration gradientcomprises a maximum dopant concentration, a first half-maximum dopantconcentration, and a first slope at or near the first half-maximumdopant concentration, wherein an absolute value of the first slope is inthe range of about 0.001 and about 0.004 (at %/μm).

In some embodiments, the maximum dopant concentration is in the range ofabout 0.25 at % to about 0.5 at %.

In some embodiments, the maximum dopant concentration is located no morethan about 100 μm away from the first or second surface. In someembodiments, the maximum dopant concentration is located no more thanabout 100 μm away from the center of the thickness of the YAG region. Insome embodiments, the first half-maximum dopant concentration is locatedat least about 25 μm away from the location of the maximum dopantconcentration. In some embodiments, the first half-maximum dopantconcentration is located at least 50 μm away from the first and secondsurfaces.

In some embodiments, the emissive ceramic includes a second half-maximumdopant concentration located at least 25 μm away from the location ofthe first half-maximum dopant concentration. In some embodiments, thesecond half-maximum dopant concentration is located at least 25 μm awayfrom the location of the maximum dopant concentration.

In some embodiments, the emissive ceramic includes a second slope at ornear the second half-maximum dopant concentration, wherein an absolutevalue of the second slope is in the range of 0.001 and 0.004 (at %/μm).In some embodiments, the absolute value of the first slope is about thesame as the absolute value of the second slope.

In some embodiments, the dopant concentration gradient comprises a peakhaving a full-width at half-maximum in the range of about 50 μm to about400 μm.

In some embodiments, the thickness of the YAG region is in the range ofabout 100 μm to about 1 mm.

In some embodiments, the YAG region further comprises a first porousregion. In some embodiments, the first porous region has a pore volumein the range of about 0.5% to about 80%. In some embodiments, the firstporous region has a pore volume in the range of about 1% to about 30%.

In some embodiments, the first porous region comprises pores having anaverage size in the range of about 0.5 μm to about 50 μM. In someembodiments, the pores have an average size in the range of about 1.0 μmto about 10 μm.

In some embodiments, the YAG region further comprises a first non-porousregion and a second non-porous region, and wherein the first porousregion is disposed between the first non-porous region and the secondnon-porous region.

In some embodiments, the YAG region further comprises a first non-porousregion and a second porous region, and wherein the first non-porousregion is disposed between the first porous region and the second porousregion.

In some embodiments, the first porous region is located no more thanabout 100 μm away from the center of the thickness of the YAG region.

In some embodiments, the first porous region is located at least about25 μm away from the center of the thickness of the YAG region.

In some embodiments, the first porous region is located at or near thefirst surface of the YAG region or the second surface of the YAG region.

In some embodiments, the YAG region is about the same size as the porousregion.

In some embodiments, the first porous region has a thickness in therange of about 10 μm to about 400 μm.

In some embodiments, the first porous region is obtained by volatilizingorganic particles within the YAG region or a precursor thereof.

Some embodiments disclosed herein include an emissive ceramic comprisinga yttrium aluminum garnet (YAG) region and a dopant having aconcentration gradient along a thickness of the YAG region, wherein saidconcentration gradient comprises a maximum dopant concentration, a firsthalf-maximum dopant concentration, and a first slope at or near thefirst half-maximum dopant concentration, wherein an absolute value ofthe first slope is in the range of about one-eighth of the maximumdopant concentration divided by the thickness and about two times themaximum dopant concentration divided by the thickness.

In some embodiments, the maximum dopant concentration is located betweenabout one-quarter and about three-quarters along the thickness of theYAG region. In some embodiments, the maximum dopant concentration islocated at or near the center of the thickness of the YAG region.

In some embodiments, the maximum dopant concentration is located no morethan one-tenth of the thickness apart from a first or second surface onopposite sides of the thickness of the YAG region.

In some embodiments, the maximum dopant concentration is in the range ofabout 0.25 at % to about 0.5 at %. In some embodiments, the first slopeis in the range of 0.001 and 0.004 (at %/μm).

In some embodiments, said dopant concentration further comprises asecond slope at or near a second half-maximum dopant concentration,wherein an absolute value of the second slope is in the range of aboutone-eighth of the maximum dopant concentration divided by the thicknessand about two times the maximum dopant concentration divided by thethickness.

In some embodiments, the absolute value of the first and second slopesis about the same.

In some embodiments, the first half-maximum dopant concentration is atleast about one-tenth of the thickness of the YAG region apart from boththe first surface and the second surface, and wherein the firsthalf-maximum dopant concentration is at least about one-tenth of thethickness apart from the maximum dopant concentration.

In some embodiments, the second half-maximum dopant concentration is atleast about one-tenth of the thickness of the YAG region apart from boththe first surface and the second surface, and wherein the secondhalf-maximum dopant concentration is at least about one-tenth of thethickness apart from both the maximum dopant concentration and the firsthalf-maximum concentration.

In some embodiments, the thickness of the YAG region is in the range ofabout 100 μm to about 1 mm.

In some embodiments, said dopant concentration gradient is generallysymmetric about a point at or near the center of the YAG region.

In some embodiments, said dopant concentration gradient includes dopantconcentrations that are effective to produce luminescence alongsubstantially all of the thickness of the YAG region.

In some embodiments, dopant concentration gradient has an average dopantconcentration in the range of about 0.01 at % to about 0.5 at %.

In some embodiments, a ratio of the maximum dopant concentration to theaverage dopant concentration is in the range of about 5:1 to about1.5:1.

In some embodiments, said dopant concentration gradient furthercomprises a peak having a full-width at half-maximum in the range ofabout one-fifth of the thickness to about four-fifths of the thickness.

In some embodiments, the YAG region further comprises a first porousregion.

In some embodiments, the first porous region has a pore volume in therange of about 0.5% to about 80%.

In some embodiments, the first porous region has a pore volume in therange of about 10% to about 30%.

In some embodiments, the first porous region comprises pores having anaverage size in the range of about 0.5 μm to about 50 μm. In someembodiments, the pores have an average size in the range of about 1.0 μmto about 10 μm.

In some embodiments, the YAG region further comprises a first non-porousregion and a second non-porous region, and wherein the first porousregion is disposed between the first non-porous region and the secondnon-porous region.

In some embodiments, the YAG region further comprises a first non-porousregion and a second porous region, and wherein the first non-porousregion is disposed between the first porous region and the second porousregion.

In some embodiments, the first porous region is located no more thanabout 100 μm away from the center of the thickness of the YAG region.

In some embodiments, the first porous region is located at least about25 μm away from the center of the thickness of the YAG region.

In some embodiments, the first porous region is located at or near thefirst surface of the YAG region or the second surface of the YAG region.

In some embodiments, the YAG region is about the same size as the porousregion.

In some embodiments, the first porous region has a thickness in therange of about 10 μm to about 400 μm.

In some embodiments, the first porous region is obtained by volatilizingorganic particles within the YAG region or a precursor thereof.

In some embodiments, the emissive ceramic exhibits an internal quantumefficiency (IQE) of at least about 0.80 when exposed to radiation havinga wavelength of about 455 nm.

Some embodiments disclosed herein include a method of forming anemissive ceramic comprising sintering an assembly, wherein the assemblycomprises a doped layer disposed on one side of a first non-doped layer,wherein: the doped layer comprises yttrium aluminum garnet (YAG), a YAGprecursor, or combination thereof, and a dopant, wherein the doped layerhas a thickness in the range of about 10 μm to about 200 μm; the firstnon-doped layers comprises YAG, a YAG precursor, or combination thereof,wherein the first non-doped layer has a thickness in the range of about40 μm to about 800 μm; and at least about 30% of the dopant in the dopedlayer diffuses out of the doped layer during said process.

In some embodiments, the doped layer is disposed between the firstnon-doped layer and a second non-doped layer comprising YAG, a YAGprecursor, or combination thereof, and wherein the first and secondnon-doped layers each independently have a thickness in the range ofabout 40 μm to about 400 μm.

In some embodiments, at least a portion of the dopant in the doped layerdiffuses into both the first and the second non-doped layers during thesintering.

In some embodiments, no more than about 80% of the dopant in the dopedlayer diffuses out of the doped layer during said process.

In some embodiments, the doped layer has a thickness in the range ofabout 40 μm to about 80 μm.

In some embodiments, the thickness of the first non-doped layer is aboutthe same the thickness of the second non-doped layer; and both thethickness of the first non-doped layer and the thickness of the secondnon-doped layer is greater than the thickness of the doped layer.

In some embodiments, the thickness of the first non-doped layer is lessthan the thickness of the second non-doped layer; and the thickness ofthe second non-doped layer is greater than or equal to the thickness ofthe doped layer.

In some embodiments, the assembly has a total thickness in the range ofabout 100 μm to about 1 mm.

In some embodiments, the doped layer comprises about 0.1 at % to about 5at %.

In some embodiments, a first amount of dopant in the doped layerdiffuses into the first non-doped layer during said process; a secondamount of dopant in the doped layer diffuses into the second non-dopedlayer during said process; and the ratio of the first amount of dopantto the second amount of dopant is in the range of about 4:1 to about1:4.

In some embodiments, the first amount of dopant is about the same as thesecond amount of dopant.

In some embodiments, sintering said assembly comprises heating theassembly at a temperature in the range of about 1000° C. to about 1900°C. for at least about 2 hrs.

In some embodiments, said temperature is in the range of about 1300° C.to about 1800° C.

In some embodiments, the assembly is heated at said temperature for atleast about 5 hours.

In some embodiments, the assembly is heated at said temperature for nomore than about 20 hours.

In some embodiments, at least one of the doped layer or the firstnon-doped layer comprises organic particles.

In some embodiments, the organic particles comprise a polymer.

In some embodiments, the organic particles have a largest diameter inthe range of about 0.5 μm to about 50 μm.

In some embodiments, said at least one of the doped layer and the firstnon-doped layer comprises an amount of organic particles in the range ofabout 0.5% by volume to about 80% by volume.

In some embodiments, the doped layer comprises the organic particles. Insome embodiments, the first non-doped layer comprises the organicparticles. In some embodiments, each layer of the assembly comprises theorganic particles. In some embodiments, at least one layer in theassembly is substantially free of the organic particles.

Some embodiments disclosed herein include an emissive ceramic made bythe any of the methods disclosed herein.

In some embodiments, the emissive ceramic exhibits an internal quantumefficiency (IQE) of at least about 0.80 when exposed to radiation havinga wavelength of about 455 nm.

Some embodiments disclosed herein include a lighting apparatuscomprising: a light source configured to emit blue radiation; and any ofthe emissive ceramics disclosed herein, wherein the emissive ceramic isconfigured to receive at a least a portion of the blue radiation.

Some embodiments disclosed herein include a method of producing lightcomprising exposing any one of the emissive ceramics disclosed herein toa blue radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C illustrate non-limiting examples of emissive ceramics.

FIG. 2 is a graph showing a prior art example of a dopant concentrationprofile along the thickness of an yttrium aluminum garnet region.

FIG. 3 is a graph showing one embodiment of a dopant concentrationgradient that is within the scope of the present application.

FIG. 4 is a graph showing another embodiment of a dopant concentrationgradient that is also within the scope of the present application.

FIGS. 5A-B illustrate one embodiment of an assembly that may be sinteredaccording to the methods disclosed herein.

FIGS. 6A-B illustrate another embodiment of an assembly that may besintered according to the methods disclosed herein.

FIG. 7 shows a preparation flow diagram for one embodiment of formingthe emissive ceramic that includes lamination.

FIG. 8 is an example of a lighting apparatus that may include theemissive ceramics disclosed herein.

FIG. 9 is an example of how the Internal Quantum Efficiency (IQE) can bedetermined.

FIG. 10 shows TOF-SIMS (Time-Of-Flight Secondary Ion Mass Spectroscopy)results for the Sample 1 in Example 5.

FIG. 11 shows TOF-SIMS (Time-Of-Flight Secondary Ion Mass Spectroscopy)results for the Sample 2 in Example 5.

FIG. 12 shows TOF-SIMS (Time-Of-Flight Secondary Ion Mass Spectroscopy)results for the Sample 3 in Example 5.

FIG. 13 shows IQE variation with emissive layer thickness in sampleswith a sandwich structure having varied dopant concentrations (Ce) inthe range of 0.2 to 1.25 at %.

FIG. 14 shows IQE variation with emissive layer thickness in sampleswith a sandwich structure having varied dopant concentrations (Ce) inthe range of 1.25 to 2.0 at %.

FIG. 15 is a graph showing IQE variation with dopant (Ce) concentrationin YAG:Ce ceramics with a sandwich structure.

FIG. 16 shows a comparative sample of a structure before sintering thatmultiple doped sheets having different dopant (Ce) concentrations.

FIG. 17 shows intensity versus wavelength of luminescence of samples inExample 10.

FIG. 18 shows a calibration curve, in which the ratio of cerium ionintensity (Ce+/Y+) in atomic percent was plot with changing Ce+concentration, was prepared by using standard samples with different Ce+concentration for a quantitative analysis.

FIG. 19 shows the respective TOF-SIMS profile illustrating the diffusionof Ce % through the emissive region (Ce concentration [%] along thethickness [μm]) for Sample 7 of Example 12.

FIG. 20 shows the respective TOF-SIMS profile illustrating the diffusionof Ce % through the emissive region (Ce concentration [%] along thethickness [μm]) for Sample 6 of Example 12.

FIG. 21 shows the respective TOF-SIMS profile illustrating the diffusionof Ce % through the emissive region (Ce concentration [%] along thethickness [μm]) for Sample 5 of Example 12.

FIG. 22 shows the respective TOF-SIMS profile illustrating the diffusionof Ce % through the emissive region (Ce concentration [%] along thethickness [μm]) for Sample 8 of Example 12.

FIG. 23 shows the respective TOF-SIMS profile illustrating the diffusionof Ce % through the emissive region (Ce concentration [%] along thethickness [μm]) for Sample 9 of Example 12.

FIG. 24 shows the respective TOF-SIMS profile illustrating the diffusionof Ce % through the emissive region (Ce concentration [%] along thethickness [μm]) for Sample 10 of Example 12.

FIGS. 25A-C illustrate non-limiting examples of emissive ceramics havingone or more porous regions.

FIG. 26 shows the setup for measuring the angular dependence ofchromaticity for emissive ceramics.

FIG. 27 shows the results for measuring the angular dependence ofchromaticity. The squares represent measurements for the emissiveceramic prepared in Example 15. The circles represent measurement forthe emissive ceramic prepared in Example 11.

DETAILED DESCRIPTION

Disclosed herein are emissive ceramic materials that can providesuperior internal quantum efficiency by including a dopant concentrationgradient along at least one dimension. Applicants have surprisinglydiscovered a dopant concentration profile, under certain conditions,provides superior internal quantum efficiency. The emissive ceramics mayoptionally include one or more porous regions. Also disclosed herein aremethods of making emissive ceramics that include a dopant concentrationprofile. In addition, the present application includes lightingapparatuses having the emissive ceramic material and methods of usingthe emissive ceramic material.

Emissive Ceramics

Some embodiments disclosed herein include an emissive ceramic having anyttrium aluminum garnet (YAG) region and a dopant having a concentrationgradient along the thickness of the YAG region. The emissive ceramicmay, for example, be prepared by sintering an assembly. Therefore, an“emissive ceramic” generally describes the final emissive material thatcan be used for lighting purposes, while an “assembly” is a compositethat may be sintered to form the emissive ceramic. As will be discussedfurther below, sintering an assembly is one method for causing dopantdiffusion that forms a dopant concentration gradient within the finalemissive ceramic.

The emissive ceramics may include a dopant concentration gradient along“a thickness of a YAG region.” FIGS. 1A-C show the thickness of the YAGregion along the z-axis for various non-limiting examples of theemissive ceramic. FIG. 1A illustrates one non-limiting example of anemissive ceramic. Emissive ceramic 100 may contain a generally uniformdistribution of an emissive dopant material within a yttrium aluminumgarnet host material and includes a length, width, and thickness alongthe x-axis, y-axis, and z-axis, respectively. In some embodiments, thethickness of the YAG region is the smallest dimension of the emissiveceramic. For example, the emissive ceramic may have both the length andwidth equal to about 1 mm, and a thickness of about 100 μm. In someembodiments, the thickness is not the smallest dimension. Of course,numerous other geometries are within the scope of the presentapplication, which is not limited to the generally cuboid geometryportrayed in FIG. 1A. For example, the emissive ceramic may also becylindrical, cubic, etc. Furthermore, there is no requirement that theemissive ceramic is symmetric, include clearly defined edges or aspecific number of faces.

FIG. 1B illustrates another embodiment of an emissive ceramic. Emissiveceramic 110 may contain a generally uniform dispersion of yttriumaluminum garnet and has a cylindrical or disk-like geometry. Emissiveceramic 110 includes a thickness along the z-axis.

FIG. 1C illustrates some embodiments of an emissive ceramic that doesnot include a generally uniform dispersion of yttrium aluminum garnetthroughout the emissive ceramic. Emissive ceramic 115 includes anemissive yttrium aluminum garnet region 120 interposed between twonon-YAG regions 130 and 140 (e.g., region consisting of aluminum oxide).Interface 150 is between emissive yttrium aluminum garnet region 120 andnon-YAG region 140. Similarly, interface 160 is between emissive yttriumaluminum garnet region 120 and non-YAG region 130. In this embodiment,the thickness of the yttrium aluminum garnet region does not includeportions of non-YAG regions 130 and 140. The thickness of yttriumaluminum garnet region 120 is between surface 150 and surface 160 alongthe z-axis. The thickness of yttrium aluminum garnet region 120 istherefore less than the thickness of emissive ceramic 115.

FIG. 2 is a graph showing a prior art example of a dopant concentrationprofile along the thickness of an emissive yttrium aluminum garnetregion. The horizontal axis is the position along the thickness of theemissive yttrium aluminum garnet region (e.g., along the z-axis ofemissive ceramic 100 illustrated in FIG. 1A). The position ‘0’ along thehorizontal axis would be at a first surface of the emissive layer (e.g.,the bottom face of emissive ceramic 100). The position ‘½L’ along thehorizontal axis would be at the midpoint along the thickness (e.g., atabout the center along the thickness of emissive ceramic 100). Theposition ‘L’ on the horizontal axis would be at a second surface of theemissive layer that is opposite to the first surface along thickness(e.g., the top face of emissive ceramic 100). Accordingly, the firstsurface and the second surface are on opposite sides of the thickness(e.g., the bottom face and top face of emissive ceramic 100 are onopposite sides along the z-axis).

Meanwhile, the vertical axis in FIG. 2 is the dopant concentration at agiven position along the thickness. This prior art dopant concentrationprofile has several notable features. First, the example generallyincludes a substantially constant dopant concentration profile along thethickness (i.e., the slope is about 0 along the entire thickness).Second, the dopant concentration profile has a maximum dopantconcentration (C_(max)) found along substantially all of the thickness.Third, the minimum dopant concentration (C_(min)) is about the same asthe maximum dopant concentration. Fourth, the dopant concentration isgreater than half the maximum dopant concentration along substantiallyall of the thickness. In other words, the substantially constant dopantconcentration profile does not include a half-maximum dopantconcentration (C_(max/2)).

FIG. 3 is a graph showing one embodiment of a dopant concentrationgradient that is within the scope of the present application. A maximumdopant concentration is located at or near a first surface of theyttrium aluminum garnet region, while a minimum dopant concentration islocated at or near a second surface that is opposite to the firstsurface along the thickness. Also, a half-maximum dopant concentrationis located at or near the center of the yttrium aluminum garnet region.

In contrast to the substantially constant dopant concentration profilein FIG. 2, the dopant concentration gradient depicted in FIG. 3 has avarying slope that includes non-zero values. As used herein, the term“slope” means the rate of change of the concentration relative to thechange in position along the thickness (e.g., horizontal axis in FIG.2). If the dopant concentration can be reasonably represented by acontinuous function f(x), then the slope may be determined from thederivative of this function. As an example, the absolute value of theslope at about ½L (or at about the half-maximum dopant concentration) inFIG. 3 is more than about the maximum dopant concentration divided bythe thickness of the yttrium aluminum garnet region (C_(max)/L).

FIG. 4 is a graph showing another embodiment of a dopant concentrationgradient that is also within the scope of the present application. Amaximum dopant concentration is at or near the center of the yttriumaluminum garnet region, while minimum dopant concentrations are locatedat or near the first and the second surfaces that are on opposite sidesalong the thickness. Also, two half-maximum dopant concentrations arelocated at about one-quarter of L (i.e., ¼L) and three-quarters of L. Inthis embodiment, both half-maximum dopant concentrations have slopeswith about the same magnitude. The dopant concentration gradient alsoexhibits non-zero values (or more than trace amounts) for the dopantconcentration along substantially all of the thickness.

The dopant concentration gradient for the embodiment shown in FIG. 4 isalso generally symmetric about the center of the yttrium aluminum garnetregion. In other words, if the dopant concentration gradient could bereasonably represented by a continuous function f(x), then f(½L−z) wouldbe about the same as f(½L+z) for 0≦z≦½L. The dopant concentrationgradient may also be characterized by a single peak. This peak has afull-width at half-maximum (e.g., the distance between the twohalf-maximum dopant concentrations along the thickness) of about ½L(i.e., ¾L minus ¼L)

Another feature that may be used to characterize the dopantconcentration gradient is the average dopant concentration. The averagedopant concentration may be determined by summing a series of evenlydistributed concentrations along the entire thickness and dividing thissum by the total number of concentrations used in the summation.Alternatively, if the dopant concentration gradient can be reasonablyrepresented by a continuous function f(x), the average dopantconcentration may be equal to

$\int_{0}^{L}{\frac{{f(x)}\ {\mathbb{d}x}}{L}.}$As a simple example, the average dopant concentration for the profileillustrated in FIG. 2 would be the maximum dopant concentration. Incontrast, both FIGS. 3 and 4 illustrate average concentration profilesthat are less than the maximum dopant concentration.

As would be appreciated by those skilled in the art, FIGS. 2-4 representsimplified illustrations of dopant concentration profiles. Variousfactors should be considered when comparing these profiles withexperimental results. For example, one may need to account for noisethat distorts the measured dopant concentration. This may requiremanipulating the data to reduce distortions using various methods knownin the art (e.g., using Fourier transforms, averaging measurements,etc.). Similarly, additional distortions may arise due tosurface-effects or minor defects in the material. The skilled artisancan recognize these distortions and appreciate that these differencesare not distinguishing features from the illustrated dopantconcentration gradients disclosed herein.

Maximum Dopant Concentration

Some embodiments of the emissive ceramic include a dopant concentrationgradient having a maximum dopant concentration. Applicants have foundthe maximum dopant concentration may correlate with the internal quantumefficiency. Without being bound to any particular theory, it is believedthat a high maximum dopant concentration results in excessive quenchingthat diminishes efficiency. Furthermore, a low maximum dopantconcentration reduces efficiency because there are a reduced number ofemissive centers, which in terms limits the ability to create photons.

Thus, in some embodiments, the maximum dopant concentration is in therange of about 0.1 atomic % (at %) to about 1 at %. The maximum dopantconcentration may, for example, be at least about 0.1 at %; at leastabout 0.2 at %; at least about 0.25 at %; at least about 0.275 at %; atleast about 0.28 at %; or at least about 0.3 at %. The maximum dopantconcentration may, for example, be no more than about 1 at %; no morethan about 0.75 at %; no more than about 0.5 at %; no more than about0.4 at %; or no more than about 0.3 at %.

The maximum dopant concentration may be located at one or more positionsalong the thickness of the yttrium aluminum garnet region. In someembodiments, the maximum dopant concentration is located apart from afirst and second surface on opposite sides of the thickness. Forexample, the maximum dopant concentration may be location between aboutone-quarter of the thickness and about three-quarters of the thicknessalong the thickness of the yttrium aluminum garnet region. As anexample, if the thickness of the yttrium aluminum garnet region is about100 μm, then the maximum dopant concentration may be located betweenabout 25 μm and about 75 μm along the thickness.

The maximum dopant concentration may, in some embodiments, be located atposition along the thickness that is at least about one-third of thethickness; at least about three-eighths of the thickness; at least abouttwo-fifths of the thickness; at least about three-sevenths of thethickness; or at least about four-ninths of the thickness. The maximumdopant concentration may, in some embodiments, be located at positionalong the thickness that is at no more than about two-thirds of thethickness; no more than about five-eighths of the thickness; no morethan about three-fifths of the thickness; no more than aboutfour-sevenths of the thickness; or no more than about five-ninths of thethickness.

The maximum dopant concentration may also be located no more than aspecified distance apart from the midpoint along the thickness. As oneexample, the maximum dopant concentration may be located no more thanabout 250 μm apart from the midpoint along the thickness. The maximumdopant concentration may, in some embodiments, be located no more thanabout 200 μm apart from the midpoint along the thickness; no more thanabout 100 μm apart from the midpoint along the thickness; no more thanabout 50 μm apart from the midpoint along the thickness; no more thanabout 25 μm apart from the midpoint along the thickness; or no more thanabout 10 μm apart from the midpoint along the thickness. In someembodiments, the maximum dopant concentration may, in some embodiments,be located no more than about three-eighths of the thickness apart fromthe midpoint along the thickness; no more than about one-quarter of thethickness apart from the midpoint along the thickness; no more thanabout one-fifth of the thickness apart from the midpoint along thethickness; no more than about one-eighth of the thickness apart from themidpoint along the thickness; or no more than about one-tenth of thethickness apart from the midpoint along the thickness. In someembodiments, the maximum dopant concentration is located at or near thecenter of the yttrium aluminum garnet region.

The maximum dopant concentration may, in some embodiments, be located nomore than a specified distance apart from the first or second surface.The maximum dopant concentration may, in some embodiments, be located nomore than about 200 μm apart from the first or second surface; no morethan about 100 μm apart from the first or second surface; no more thanabout 50 μm apart from the first or second surface; no more than about25 μm apart from first or second surface; or no more than about 10 μmapart from the first or second surface. In some embodiments, the maximumdopant concentration is located at or near the first or second surface.

First Half-Maximum Dopant Concentration

The dopant concentration gradient may also include a first half-maximumdopant concentration. The half-maximum dopant concentration refers to adopant concentration that is half of the maximum dopant concentrationfor a particular emissive ceramic. For example, in one embodiment, thedopant concentration gradient may have a maximum dopant concentration ofabout 0.5 at % and a half-maximum dopant concentration of about 0.25 at%.

The first half-maximum dopant concentration may, in some embodiments, beat a certain distance apart from the first and second surfaces along thethickness. The first half-maximum dopant concentration may, for example,be located at least 10 μm apart from the first and second surfaces; atleast about 25 μm apart from the first and second surfaces; at leastabout 50 μm apart from the first and second surfaces; at least about 100μm apart from the first and second surfaces; at least about 200 μm apartfrom the first and second surfaces; or at least about 250 μm apart fromthe first and second surfaces. The first half-maximum dopantconcentration may, for example, be located at least one-tenth of thethickness apart from the first and second surfaces; at least aboutone-eighth of the thickness apart from the first and second surfaces; atleast about one-fifth of the thickness apart from the first and secondsurfaces; at least about one-quarter of the thickness apart from thefirst and second surfaces; or at least about three-eighths of thethickness apart from the first and second surfaces.

The first half-maximum dopant concentration can also be at a certaindistance apart from the maximum dopant concentration. The firsthalf-maximum dopant concentration may, for example, be at least 10 μmapart from the maximum dopant concentration; at least about 25 μm apartfrom the maximum dopant concentration; at least about 50 μm apart fromthe maximum dopant concentration; at least about 100 μm apart from themaximum dopant concentration; at least about 200 μm apart from themaximum dopant concentration; or at least about 250 μm apart from themaximum dopant concentration. The first half-maximum dopantconcentration may, for example, be located at least one-tenth of thethickness apart from the maximum dopant concentration; at least aboutone-eighth of the thickness apart from the maximum dopant concentration;at least about one-fifth of the thickness apart from the maximum dopantconcentration; at least about one-quarter of the thickness apart fromthe maximum dopant concentration; or at least about three-eighths of thethickness apart from the maximum dopant concentration.

In some embodiments, the dopant concentration gradient includes a firstslope at or near the first half-maximum dopant concentration. Theabsolute value of the first slope may, for example, be in range of aboutone-eighth of the maximum dopant concentration divided by the thickness(i.e., C_(max) divided by 8L) and about two times the maximum dopantconcentration divided by the thickness. As an example, if the maximumdopant concentration is about 0.2 at % and the thickness is about 100μm, then the absolute value of the first slope would be in the range ofabout 0.00025 at %/μm and about 0.004 at %/μm.

The absolute value of the first slope may, for example, be at leastabout one-quarter of the maximum dopant concentration divided by thethickness; at least about three-eighths of the maximum dopantconcentration divided by the thickness; at least about one-half of themaximum dopant concentration divided by the thickness; at least aboutthree-quarters of the maximum dopant concentration divided by thethickness; or at least about the maximum dopant concentration divided bythe thickness.

The absolute value of the first slope may, for example, be no more thanabout the maximum dopant concentration divided by the thickness; no morethan about three-quarters of the maximum dopant concentration divided bythe thickness; no more than about one-half of the maximum dopantconcentration divided by the thickness; or no more than aboutthree-eighths of the maximum dopant concentration divided by thethickness.

The absolute value of the first slope may also be within a certainnumerical range. For example, the absolute value of the first slope maybe in the range of about 0.001 at %/μm and about 0.004 at %/μm. Theabsolute value of the first slope may, in some embodiments, be at leastabout 0.0001 at %/μm; at least about 0.0005 at %/μm; at least about0.001 at %/μm; at least about 0.0015 at %/μm; or at least about 0.002 at%/μm. The absolute value of the first slope may, in some embodiments, nomore than about 0.006 at %/μm; no more than about 0.005 at %/μm; no morethan about 0.004 at %/μm; or no more than about 0.003 at %/μm.

Second Half-Maximum Dopant Concentration

The dopant concentration gradient may also include a second half-maximumdopant concentration. The second half-maximum dopant concentration may,in some embodiments, be located at a certain distance apart from thefirst and second surfaces along the thickness. The second half-maximumdopant concentration may, for example, be located at least 10 μm apartfrom the first and second surfaces; at least about 25 μm apart from thefirst and second surfaces; at least about 50 μm apart from the first andsecond surfaces; at least about 100 μm apart from the first and secondsurfaces; at least about 200 μm apart from the first and secondsurfaces; or at least about 250 μm apart from the first and secondsurfaces. The second half-maximum dopant concentration may, for example,be located at least one-tenth of the thickness apart from the first andsecond surfaces; at least about one-eighth of the thickness apart fromthe first and second surfaces; at least about one-fifth of the thicknessapart from the first and second surfaces; at least about one-quarter ofthe thickness apart from the first and second surfaces; or at leastabout three-eighths of the thickness apart from the first and secondsurfaces.

The second half-maximum dopant concentration may, for example, belocated at least 10 μm apart from the maximum dopant concentration; atleast about 25 μm apart from the maximum dopant concentration; at leastabout 50 μm apart from the maximum dopant concentration; at least about100 μm apart from the maximum dopant concentration; at least about 200μm apart from the maximum dopant concentration; or at least about 250 μmapart from the maximum dopant concentration. The second half-maximumdopant concentration may, for example, be located at least one-tenth ofthe thickness apart from the maximum dopant concentration; at leastabout one-eighth of the thickness apart from the maximum dopantconcentration; at least about one-fifth of the thickness apart from themaximum dopant concentration; at least about one-quarter of thethickness apart from the maximum dopant concentration; or at least aboutthree-eighths of the thickness apart from the maximum dopantconcentration.

In some embodiments, the second half-maximum dopant concentration islocated at least a specified distance apart from the first half-maximumdopant concentration along the thickness. The second half-maximum dopantconcentration may, for example, be located at least 10 μm apart from thefirst half-maximum dopant concentration; at least about 25 μm apart fromthe first half-maximum dopant concentration; at least about 50 μm apartfrom the first half-maximum dopant concentration; at least about 100 μmapart from the first half-maximum dopant concentration; at least about200 μm apart from the first half-maximum dopant concentration; or atleast about 250 μm apart from the first half-maximum dopantconcentration. The second half-maximum dopant concentration may, forexample, be located at least one-tenth of the thickness apart from thefirst half-maximum dopant concentration; at least about one-eighth ofthe thickness apart from the first half-maximum dopant concentration; atleast about one-fifth of the thickness apart from the first half-maximumdopant concentration; at least about one-quarter of the thickness apartfrom the first half-maximum dopant concentration; or at least aboutthree-eighths of the thickness apart from the first half-maximum dopantconcentration.

In some embodiments, the dopant concentration gradient includes a secondslope at or near the second half-maximum dopant concentration. Theabsolute value of the second slope may, for example, be in range ofabout one-eighth of the maximum dopant concentration divided by thethickness (i.e., C_(max) divided by 8L) and about two times the maximumdopant concentration divided by the thickness. As an example, if themaximum dopant concentration is about 0.2 at % and the thickness isabout 100 μm, then the absolute value of the second slope would be inthe range of about 0.00025 at %/μm and about 0.004 at %/μm.

The absolute value of the second slope may, for example, be at leastabout one-quarter of the maximum dopant concentration divided by thethickness; at least about three-eighths of the maximum dopantconcentration divided by the thickness; at least about one-half of themaximum dopant concentration divided by the thickness; at least aboutthree-quarters of the maximum dopant concentration divided by thethickness; or at least about the maximum dopant concentration divided bythe thickness.

The absolute value of the second slope may, for example, be no more thanabout the maximum dopant concentration divided by the thickness; no morethan about three-quarters of the maximum dopant concentration divided bythe thickness; no more than about one-half of the maximum dopantconcentration divided by the thickness; or no more than aboutthree-eighths of the maximum dopant concentration divided by thethickness.

The absolute value of the second slope may also be within a certainnumerical range. For example, the absolute value of the second slope maybe in the range about 0.001 at %/μm and about 0.004 at %/μm. Theabsolute value of the second slope may, in some embodiments, be at leastabout 0.0001 at %/μm; at least about 0.0002 at %/μm; at least about0.0005 at %/μm; at least about 0.001 at %/μm; or at least about 0.002 at%/μm. The absolute value of the second slope may, in some embodiments,be no more than about 0.01 at %/μm; no more than about 0.008 at %/μm; nomore than about 0.004 at %/μm; no more than about 0.002 at %/μm; or nomore than about 0.001 at %/μm.

In some embodiments, the absolute value of the first slope is about thesame as the absolute value of the second slope.

Time-Of-Flight Secondary Ion Mass Spectroscopy can be used, for example,to determine the dopant concentration, thickness or distances/locationsof the respective points of interest, and the resultant slopes at therespective points of interest.

Other Characteristics

The thickness of the yttrium aluminum garnet region is not particularlylimited. In some embodiments, the thickness of the yttrium aluminumgarnet region is in the range of about 100 μm to about 1 mm. Thethickness of the yttrium aluminum garnet region may, for example, be atleast about 100 μm; at least about 150 μm; at least about 200 μm; atleast about 250 μm; at least about 400 μm; or at least about 500 μm. Thethickness of the yttrium aluminum garnet region may, for example, be nomore than about 1 mm; no more than about 900 μm; no more than about 800μm; no more than about 750 μm; no more than about 600 μm; or no morethan about 500 μm.

Some embodiments of the dopant concentration gradient are generallysymmetric about a point at or near the center of the yttrium aluminumgarnet regions (e.g., as depicted in FIG. 4). Some embodiments of thedopant concentration gradient are generally asymmetric about a point ator near the center of the yttrium aluminum garnet regions (e.g., asdepicted in FIG. 3). Other embodiments can be asymmetric about a pointthat is apart from the center of the yttrium aluminum garnet regions,e.g., with C_(max) between 0 to ½L, such as ¼L, or in another embodimentwith C_(max) between ½L to L, such as ¾L (not shown).

The dopant concentration gradient, in some embodiments, may include anon-zero dopant concentration along substantially all of the thickness.That is, the dopant concentration is greater than zero alongsubstantially all of the thickness. In some embodiments, the dopantconcentration gradient may include dopant concentrations that areeffective to produce luminescence along substantially all of thethickness.

The dopant concentration gradient may include a peak having a specifiedfull-width at half-maximum. For example, the full-width at half-maximummay be in the range of about 50 μm to about 500 μm. The full-width athalf-maximum may, for example, be at least about 50 μm; at least about75 μm; at least about 100 μm, at least about 150 μm; or at least about200 μm. The full-width at half-maximum may, for example, be no more thanabout 500 μm; no more than about 400 μm; no more than about 300 μm; nomore than about 250 μm; no more than about 200 μm; or no more than about150 μm. In some embodiments, the full-width at half-maximum may be atleast about one-tenth of the thickness of the YAG region; at least aboutone-quarter of the thickness of the YAG region; at least aboutone-eighth of the thickness of the YAG region; at least aboutone-quarter of the thickness of the YAG region; or at least aboutthree-eighths of the thickness of the YAG region. In some embodiments,the full-width at half-maximum may be no more than about five-eighths ofthe thickness of the YAG region; no more than about one-half of thethickness of the YAG region; no more than about three-eighths of thethickness of the YAG region; or no more than about one-quarter of thethickness of the YAG region. In some embodiments, the dopantconcentration gradient consists essentially of a single peak.

Furthermore, the dopant concentration gradient may be characterized bythe average dopant concentration along the entire thickness. The averagedopant concentration may, for example, be in the range of about 0.1 at %to about 0.5 at %. In some embodiments, the average dopant concentrationmay be at least about 0.05 at %; at least about 0.1 at %; at least about0.2 at %; or at least about 0.25 at %. In some embodiments, the averagedopant concentration may be no more than about 1 at %; no more thanabout 0.5 at %; no more than about 0.4 at %; no more than about 0.3 at%; or no more than about 0.25 at %. The ratio of the maximum dopantconcentration to the average dopant concentration may be, for example,in the range of about 5:1 to about 1.5:1; in the range of about 4:1 toabout 1.5:1; or in the range of about 3:1 to about 2:1.

Non-limiting examples of dopants that may be incorporated into theyttrium aluminum garnet region to form the dopant concentration gradientinclude Nd, Er, Eu, Cr, Yb, Sm, Tb, Ce, Pr, By, Ho, Gd, Lu, andcombinations thereof. In some embodiments, the dopant is Ce. As anexample, the doped yttrium aluminum garnet region may be represented bythe formula (Y_(1-x)Ce_(x))₃Al₅O₁₂.

As disclosed above, one advantage of the emissive ceramics disclosedherein exhibit superior internal quantum efficiencies. In someembodiments, the emissive ceramic exhibits an internal quantumefficiency (IQE) of at least about 0.80 when exposed to radiation havinga wavelength of about 455 nm. In some embodiments, the emissive ceramicexhibits an internal quantum efficiency (IQE) of at least about 0.85when exposed to radiation having a wavelength of about 455 nm. In someembodiments, the emissive ceramic exhibits an internal quantumefficiency (IQE) of at least about 0.90 when exposed to radiation havinga wavelength of about 455 nm.

Porous Regions

The emissive ceramics may optionally include one or more porous regions(e.g., zero, one, two, three, or more porous regions). Without beingbound to any particular theory, it is believed that adding porous layersreduces the angular dependence of the light emission properties for theemissive ceramic. In other words, the porous regions may reduce anyanisotropy in the light emission properties of the emissive ceramic.

The location and size of the one or more porous regions within the YAGregion is not particularly limited. In some embodiments, the YAG regionincludes at least a first porous region and a first non-porous porousregion. In some embodiments, the YAG region includes a first porousregion, a first non-porous region, and a second non-porous region. Insome embodiments, the YAG region includes a first porous region, asecond porous region, and a first non-porous region.

FIG. 25A is one example of an emissive ceramic including a porousregion. Porous layer 2505 is interposed between first non-porous layer2510 and second non-porous layer 2515. Each of porous layer 2505, firstnon-porous layer 2510, and second non-porous layer 2515 may include aYAG host material (i.e., the components together form the YAG region ofthe emissive ceramic). As depicted in FIG. 25A, the porous region may belocated at or near the center of the emissive ceramic.

In some embodiments, the emissive ceramic may include a first porousregion and a second porous region, wherein the first porous region andthe second porous region have different pore volume percentages. In someembodiments, the emissive ceramic may include a first porous region anda second porous region, wherein the first porous region and the secondporous region have different average pore sizes.

FIG. 25B is one example of an emissive ceramic including a plurality ofporous layers with different pore volumes. The emissive ceramic includesfirst porous layer 2520 which has a lower porosity (e.g., lower averagepore sizes and/or lower pore volume percentage) than second porous layer2525. Both first porous layer 2520 and second porous layer 2525 aredisposed between first non-porous layer 2530 and second non-porous layer2535. Each of first porous layer 2520, second porous layer 2525, firstnon-porous layer 2530, and second non-porous layer 2535 may include aYAG host material (i.e., the components together form the YAG region ofthe emissive ceramic).

FIG. 25C is another example of an emissive ceramic including a pluralityof porous layers with different pore volumes. The emissive ceramicincludes first porous layer 2540 disposed between second porous layer2545 and third porous layer 2550. First porous layer 2540 can have adifferent porosity (e.g., lower average pore sizes and/or lower porevolume percentage) than both second porous layer 2545 and third porouslayer 2550. In some embodiments, both second porous layer 2545 and thirdporous layer 2550 have the same porosity. In some embodiments, secondporous layer 2545 and third porous layer 2550 have different porosity.Second porous layer 2545 and third porous layer 2550 may be disposedbetween first non-porous layer 2555 and second non-porous layer 2560.Each of first porous layer 2540, second porous layer 2545, third porouslayer 2550, first non-porous layer 2555, and second non-porous layer2560 may include a YAG host material (i.e., the components together formthe YAG region of the emissive ceramic).

In some embodiments, substantially all of the YAG region in the emissiveceramic is porous. That is, the YAG region is porous throughout. In someembodiments, the YAG region has a uniform porosity along its thickness.In some embodiments, the porosity of the YAG region changes along itthickness.

The size of the pores in the one or more porous regions is notparticularly limited. The average pore size in the one or more porousregions can be, for example, at least about 0.5 μm; at least about 1 μm;at least about 2 μm; at least about 4 μm; or at least about 7 μm. Theaverage pore size in the porous region can be, for example, no more thanabout 50 μm; no more than about 25 μm; no more than about 10 μm; or nomore than about 7 μm. In some embodiments, the average size of the poresin the one or more porous regions can be in the range of about 0.5 μm toabout 50 μm. For example, the average pore size can be about 4 μm. Asnoted about, the emissive ceramic may include two or more porousregions, each having different average pore sizes.

The volume percentage of the pores in the one or more porous regions mayalso vary. The volume percentage of the pores in the one or more porousregions can be, for example, at least about 0.5%; at least about 1%; atleast about 2%; at least about 4%; at least about 10; or at least about20%. The volume percentage of the pore in the one or more porous regionscan be, for example, no more than about 80%; no more than about 50%; nomore than about 30%; no more than about 20%; no more than about 10%; orno more than about 8%. In some embodiments, the volume percentage of thepore in the one or more porous regions can be in the range of about 0.5%to about 80%. For example, the volume percentage of the pore in the oneor more porous regions can be about 6%. As noted about, the emissiveceramic may include two or more porous regions, each having differentpore volume percentages.

As noted above, the one or more porous regions can be layer(s) in theemissive ceramic (e.g., porous layer 2505 depicted in FIG. 25A). Thethickness of the one or more porous layers can vary. The one or moreporous layers (e.g., one, two, three, or more porous layers) may eachindependently have a thickness in the range of about 10 μm to about 800μm. In some embodiments, the one or more porous layers may eachindependently have a thickness in the range of about 20 μm to about 400μm. The one or more porous layers may, for example, each independentlyhave a thickness that is at least about 10 μm; at least about 40 μm; atleast about 80 μm; at least about 100 μm; or at least about 200 μm. Theone or more porous layers may, for example, each independently have athickness that is no more than about 400 μm; no more than about 300 μm;no more than about 250 μm; no more than about 200 μm; no more than about150 μm; no more than about 100 μm; or no more than about 80 μm. In someembodiments, the thickness at least one porous layer (e.g., one or twoporous layers) may be less than or equal to the thickness of at leastone non-porous layer. In some embodiments, the thickness at least oneporous layer (e.g., one or two porous layers) is greater than or equalto the thickness of at least one non-porous layer.

As will described further below, the one or more porous layers can beprepared, for example, by sintering an assembly that includes organicparticles dispersed in at least a portion of the assembly. The organicparticle may be volatilized, for example, during debindering orsintering to yield pores within regions of the emissive ceramic. Unlikedopant, the organic particles exhibit minimal (if any) diffusion whenheated; therefore, the location of the porous regions correlates to thelayers in the assembly having the organic particles. Accordingly, thepore size, the pore volume percentage, and location of the porousregions may be controlled by the organic particle size, amount oforganic particles, and distribution of organic particles in theassembly. The arrangement of the porous and non-porous layers (if any)can therefore be readily varied by changing the configuration of theassembly used to prepare the emissive ceramic. Moreover, as discussedabove, the dopant concentrations in the layers of the assembly cansimilarly be varied to obtain the desired dopant and porositydistributions.

The non-porous regions can be substantially free of pores. The volumepercentage of pores in the non-porous region can be, for example, nomore than about 0.05%; no more than about 0.01%; or no more than about0.001%.

As noted above, the one or more non-porous regions can be layer(s) inthe emissive ceramic (e.g., first non-porous layer 2510 depicted in FIG.25A). The thickness of the one or more non-porous layers can vary. Theone or more non-porous layers (e.g., one, two, three, or more non-porouslayers) may each independently have a thickness in the range of about 10μm to about 800 μm. In some embodiments, the one or more non-porouslayers may each independently have a thickness in the range of about 20μm to about 400 μm. The one or more non-porous layers may, for example,each independently have a thickness that is at least about 10 μm; atleast about 40 μm; at least about 80 μm; at least about 100 μm; or atleast about 200 μm. The one or more non porous layers may, for example,each independently have a thickness that is no more than about 400 μm;no more than about 300 μm; no more than about 250 μm; no more than about200 μm; no more than about 150 μm; no more than about 100 μm; or no morethan about 80 μm.

The located of the one or more porous regions in the emissive ceramicmay optionally have a symmetric configuration in the emissive ceramic.For example, FIG. 25A depicts a symmetric emissive ceramic provided that(i) the thickness of first non-porous layer 2510 is about the same asthe thickness of second non-porous layer 2515 and (ii) the porosity isgenerally uniform in porous layer 2505. In some embodiments, the one ormore porous regions in the emissive ceramic may have an asymmetricconfiguration in the emissive ceramic. For example, FIG. 25B depicts anasymmetric emissive ceramic at least because first porous layer 2520 andsecond porous layer 2525 have different porosities.

At least one porous region may also be located no more than a specifieddistance apart from the midpoint along the thickness. As one example, atleast one porous region may be located no more than about 250 μm apartfrom the midpoint along the thickness. At least one porous region may,in some embodiments, be located no more than about 200 μm apart from themidpoint along the thickness; no more than about 100 μm apart from themidpoint along the thickness; no more than about 50 μm apart from themidpoint along the thickness; no more than about 25 μm apart from themidpoint along the thickness; or no more than about 10 μm apart from themidpoint along the thickness. In some embodiments, at least one porousregion may be located no more than about three-eighths of the thicknessapart from the midpoint along the thickness; no more than aboutone-quarter of the thickness apart from the midpoint along thethickness; no more than about one-fifth of the thickness apart from themidpoint along the thickness; no more than about one-eighth of thethickness apart from the midpoint along the thickness; or no more thanabout one-tenth of the thickness apart from the midpoint along thethickness. In some embodiments, at least one porous region is located ator near the center of the yttrium aluminum garnet region.

At least one porous layer may, in some embodiments, be located no morethan a specified distance apart from the first or second surface. Atleast one porous region may, in some embodiments, be located no morethan about 200 μm apart from the first or second surface; no more thanabout 100 μm apart from the first or second surface; no more than about50 μm apart from the first or second surface; no more than about 25 μmapart from first or second surface; or no more than about 10 μm apartfrom the first or second surface. In some embodiments, at least oneporous region is located at or near the first or second surface. In someembodiments, a first porous region is located at or near the firstsurface and a second porous region is located at or near the secondsurface.

In some embodiments, at least one non-porous region is located at ornear the first or second surface. In some embodiments, a firstnon-porous region is located at or near the first surface and a secondnon-porous region is located at or near the second surface.

In some embodiments, the maximum dopant concentration is located in aporous layer. In some embodiments, the maximum dopant concentration islocated in a non-porous layer. In some embodiments, the firsthalf-maximum dopant concentration is located in a non-porous layer. Insome embodiments, the first half-maximum dopant concentration is locatedin a porous layer. In some embodiments, the second half-maximum dopantconcentration is located in a non-porous layer. In some embodiments, thesecond half-maximum dopant concentration is located in a porous layer.In some embodiments, the maximum dopant concentration is located in aporous layer, the first half-maximum dopant concentration is located ina first non-porous layer, and the second half-maximum dopantconcentration is located in a second non-porous layer. In someembodiments, the maximum dopant concentration is located in a firstporous layer, the first half-maximum dopant concentration is located ina second porous layer, and the second half-maximum dopant concentrationis located in a third porous layer. In some embodiments, the maximumdopant concentration is located in a first porous layer, the firsthalf-maximum dopant concentration is located in a second porous layer,and the second half-maximum dopant concentration is located in a firstnon-porous layer.

Methods of Making Emissive Ceramics

Some embodiments disclosed herein include methods of forming emissiveceramics, such as any of the emissive ceramics disclosed above. Themethod may include sintering an assembly, where the assembly includes adoped layer disposed on one side of a non-doped layer.

FIGS. 5A and 5B illustrate one embodiment of an assembly that may besintered according to the methods disclosed herein. FIG. 5A is a sideview of assembly 500 having doped layer 510 disposed on one side ofnon-doped layer 520. FIG. 5B shows a perspective view of assembly 500.Assembly 500 may, for example, be configured and sintered underappropriate conditions to obtain a dopant concentration gradient similarto the one depicted in FIG. 3.

FIGS. 6A and 6B illustrate another embodiment of an assembly that may besintered according to the methods disclosed herein. FIG. 6A is a sideview of assembly 600 having doped layer 610 interposed between firstnon-doped layer 620 and second non-doped layer 630. FIG. 6B shows aperspective view of assembly 600. Assembly 600 may, for example, beconfigured and sintered under appropriate conditions to obtain a dopantconcentration gradient similar to the one depicted in FIG. 4.

The doped layer of the assembly can include a yttrium aluminum garnet,yttrium aluminum garnet precursor, or a combination thereof. A yttriumaluminum garnet precursor can be any components that will form yttriumaluminum garnet during the process. As an example, a yttrium aluminumgarnet precursor may be a mixture of Y₂O₃ and Al₂O₃ at thestoichiometric ratio of 3:5 that forms yttrium aluminum garnet duringsintering. In some embodiments, the doped layer includes at least 50%yttrium aluminum garnet and/or its equivalent amount of precursor. Insome embodiments, the doped layer includes at least 80% yttrium aluminumgarnet and/or its equivalent amount of precursor. In some embodiments,the doped layer includes at least 90% yttrium aluminum garnet and/or itsequivalent amount of precursor. In some embodiments, the doped layerconsists essentially of yttrium aluminum garnet and the desired dopant.

The doped layer also includes a dopant, such as Nd, Er, Eu, Cr, Yb, Sm,Tb, Ce, Pr, Gd, Dy, Ho, Lu and combinations thereof. In someembodiments, the dopant is Ce. The amount of dopant in the doped layercan be an amount effective to impart luminescence upon the emissiveceramic after sintering is complete. Moreover, Applicants have foundthat the initial concentration of dopant in the doped layer may affectthe internal quantum efficiency of the emissive ceramic. Thus, in someembodiments, the doped layer includes about 0.1 at % to about 5 at %dopant. The doped layer may, for example, include at least about 0.5 at%; at least about 1 at %; at least about 1.5 at %; or at least about 2at % of the dopant. The doped layer may, for example, include no morethan about 4.5 at %; no more than about 4 at %; no more than about 3.5at %; or no more than about 3 at % of the dopant. In some embodiments,the doped layer contains a generally uniform distribution of dopant.

The one or more non-doped layers in the assembly (e.g., the assemblythat can be sintered to form the emissive ceramic) can also include ayttrium aluminum garnet, yttrium aluminum garnet precursor, or acombination thereof. In some embodiments, the one or more non-dopedlayers include at least 50% yttrium aluminum garnet and/or itsprecursor. In some embodiments, the one or more non-doped layers includeat least 80% yttrium aluminum garnet and/or its equivalent amount ofprecursor. In some embodiments, the one or more non-doped layers includeat least 90% yttrium aluminum garnet and/or its equivalent amount ofprecursor. In some embodiments, the one or more non-doped layersconsists essentially of yttrium aluminum garnet and/or its equivalentamount of precursor. However, the one or more non-doped layers can besubstantially free of dopant. In some embodiments, the one or morenon-doped layers can include an amount of dopant that is ineffective forimparting luminescence upon the emissive ceramic. In some embodiments,the one or more non-doped layers can include less than about 0.05 at %dopant. In some embodiments, the one or more non-doped layers caninclude less than about 0.01 at % dopant. In some embodiments, the oneor more non-doped layers can include less than about 0.001 at % dopant.

In some embodiments, the relative thickness of the doped layer and oneor more non-doped layers may affect the internal quantum efficiencies.For example, in the case of a thinner emissive layer, dopant within thedoped layer can diffuse into the neighboring non-doped layers, whichreduces the maximum dopant concentration. Without being bound to anyparticular theory, it is believed the lower dopant concentration reducesquenching and therefore increases internal quantum efficiencies. Incontrast, a thicker emissive layer may not exhibit a similar reductionin the maximum dopant concentration because dopant is unable to diffuseacross the thicker doped layer. This in turn may result in highermaximum dopant concentrations that can increase quenching

Accordingly, in some embodiments, the doped layer has a thicknessconfigured to enable a reduction of the initial dopant concentrationwithin the doped layer. In some embodiments, the doped layer has athickness configured to enable diffusion from the doped layer and intothe non-emissive layer(s) during sintering. In some embodiments, thedoped layer has a thickness in the range of about 10 μm to about 200 μm.In some embodiments, the doped layer has a thickness in the range ofabout 40 μm to about 80 μm. The doped layer may, for example, have athickness that is at least about 20 at least about 30 μm; at least about40 μm; or at least about 50 μm. The doped layer may also, for example,have a thickness that is no more than about 150 μm; no more than about120 μm; no more than about 100 μm; no more than about 80 μm; or no morethan about 70 μm.

The one or more non-doped layers (e.g., one or two non-doped layers) mayeach independently have a thickness in the range of about 40 μm to about800 μm. In some embodiments, the one or more non-doped layers may eachindependently have a thickness in the range of about 40 μm to about 400μm. The one or more non-doped layers may, for example, eachindependently have a thickness that is at least about 40 μm; at leastabout 80 μm; at least about 100 μm; or at least about 200 μm. The one ormore non-doped layers may, for example, each independently have athickness that is no more than about 400 μm; no more than about 300 μm;no more than about 250 μm; no more than about 200 μm; no more than about150 μm. In some embodiments, at least one non-doped layer (e.g., one ortwo non-doped layers) may be less than or equal to the thickness of thedoped layer. In some embodiments, at least one non-doped layer (e.g.,one or two non-doped layers) is greater than or equal to the thicknessof the doped layer.

The assembly may, for example, consist essentially of a doped layer anda non-doped layer. In other words, the assembly includes a doped layerand non-doped layer, but excludes any other layers that form a yttriumaluminum garnet region (e.g., assembly 500 in FIGS. 5A and 5B). Thethickness of the non-doped layer may, for example, be greater than thethickness of doped layer. The ratio of the thickness of the non-dopedlayer relative to the thickness of the doped layer may, in someembodiments, be in the range of about 20:1 to about 1.5:1. The ratio ofthe thickness of the non-doped layer relative to the thickness of thedoped layer may, for example, be no more than about 15:1; no more thanabout 12:1; no more than about 10:1; no more than about 8:1; or no morethan about 5:1. The ratio of the thickness of the non-doped layerrelative to the thickness of the doped layer may also, for example, beat least about 2:1; at least about 3:1; at least about 4:1; or at leastabout 5:1.

The assembly may also include, or consist essentially of, a doped layerand two non-doped layers (e.g., assembly 600 in FIGS. 6A and 6B). Theassembly may therefore have a first non-doped layer and a secondnon-doped layer. The first and second non-doped layers can eachindependently have any thickness, such as those disclosed above. Forexample, the first non-doped layer may have a thickness in the range ofabout 40 μm to about 400 μm, and the second non-doped layer may have athickness in the range of about 40 μm to about 400 μm. In someembodiments, the first non-doped layer is thicker than the doped layer.In some embodiments, the second non-doped layer is thicker than thedoped layer. In some embodiments, both the first and second non-dopedlayers are thicker than the doped layer. In some embodiments, the firstand second non-doped layers have different thicknesses. In someembodiments, the first and second non-doped layers have thicknesses thatare about the same.

The ratio of either the first non-doped layer or the second non-dopedlayer relative to the dope layer may each independently be within theranges discussed above. For example, the ratio of the thickness of thefirst non-doped layer relative to the thickness of the doped layer canbe in the range of about 20:1 to about 1.5:1. Also, the ratio of thethickness of the second non-doped layer relative to the thickness of thedoped layer can be in the range of about 20:1 to about 1.5:1.

One or more layers in the assembly may optionally include organicparticles. These organic particles can be volatilized to form one ormore porous regions in the final emissive ceramic. In some embodiment,the organic particles can include a polymer. For example, the organicparticles can be crosslinked polymethylmethacrylate (PMMA) beads,polystyrene bead, and polyethylene bead. In some embodiments, at leastone doped layer in the assembly includes organic particles. In someembodiments, at least one non-doped layer in the assembly includesorganic particles. In some embodiments, substantially all of theassembly includes organic particles. In some embodiments, at least onelayer in the assembly is substantially free of organic particles.

The size of organic particles can be selected based on the desired poresize for the porous region in the final emissive ceramic. The averagelargest dimension of the organic particles can be, for example, at leastabout 0.5 μm; at least about 1 μm; at least about 2 μm; at least about 4μm; or at least about 7 μm. The average largest dimension of the organicparticles can be, for example, no more than about 50 μm; no more thanabout 25 μm; no more than about 10 μm; or no more than about 7 μm. Insome embodiments, the average largest dimension of the organic particlescan be in the range of about 0.5 μm to about 50 μm. For example, theaverage largest dimension of the organic particles can be about 4 μm.The assembly may, in some embodiments, include two or more layers havingorganic particles, where the average largest dimension of the organicparticles in the at least two layers in different. For example, theassembly may include a doped layer having organic particles with anaverage largest dimension of about 1 μm and a non-doped layering havingorganic particles with an average largest dimension of about 4 μm.

The organic particles may, in some embodiments, be generally spherical(e.g., beads). In some embodiments, the organic particles may have anaspect ratio less than about 10; less than about 5; or less than about2. In some embodiments, the organic particles have an aspect ratio ofabout 1.

The amount of organic particles in each of the layers may be varieddepending on the desired pore volume percentage. The amount of organicparticles necessary to obtain a desired pore volume percentage will varydepending on various factors, such as the size and density of theorganic particles. The skilled artisan, guided by the teachings of thepresent application, can readily determine this amount. In someembodiments, the amount of organic particles in at least one layer inthe assembly is configured to produce a final emissive ceramic having aporous region with a pore volume percentage in the range of about 0.5%to about 80%. In some embodiments, the amount of organic particles in atleast one layer in the assembly is configured to produce a finalemissive ceramic having a porous region with a pore volume percentage inthe range of about 1% to about 30%

Forming the Assembly

The assembly may be formed by laminating two or more cast tapes, wherethe cast tapes can include yttrium aluminum garnet. At least one of thecast tapes will also include a dopant to form the doped layer. Examplesand methods of laminating and sintering two or more cast tapes aredisclosed in U.S. Pat. No. 7,514,721 and U.S. Publication No.2009/0108507, both of which are hereby incorporated by reference intheir entirety. FIG. 7 shows a preparation flow diagram for oneembodiment of forming the emissive ceramic that includes lamination.

First, the particle size of the raw materials (e.g., nitrate or oxidebased raw materials, such as Y₂O₃ and Al₂O₃ for forming YAG) mayoptionally be adjusted to reduce cracking in the cast tapes fromcapillary forces during evaporation of solvents. For example, theparticle size can be adjusted by pre-annealing raw material particles toobtain the desired particle size. Raw material particles can bepre-annealed in the temperature range of about 800° C. to about 1800° C.(or more preferably 1000° C. to about 1500° C.) to obtain the desiredparticle size. The pre-annealing may occur in a vacuum, air, O₂, H₂,H₂/N₂, or a noble gas (e.g., He, Ar, Kr, Xe, Rn, or combinationsthereof). In an embodiment, each of the raw materials (e.g., Y₂O₃ andAl₂O₃ for forming YAG) is adjusted to be about the same particle size.In another embodiment, the particles have a BET surface area in therange of about 0.5 m²/g to about 20 m²/g (preferably about 1.0 m²/g toabout 10 m²/g, or more preferably about 3.0 m²/g to about 6.0 m²/g).

A slurry may then be prepared for subsequently casting into a tape.Pre-made phosphors (e.g., phosphors prepared by flow-basedthermochemical synthetic routes described herein) and/or stoichiometricamounts of raw materials can be intermixed with various components toform a mixture. Exemplary components for the mixture include, but arenot limited to, dopants, dispersants, plasticizers, binders, sinteringaids and solvents.

In some embodiments, small quantities of flux materials (e.g., sinteringaids) may be used in order to improve sintering properties of theassembly if desired. In some embodiments, the sintering aids mayinclude, but are not limited to, tetraethyl orthosilicate (TEOS),colloidal silica, lithium oxide, titanium oxide, zirconium oxide,magnesium oxide, barium oxide, calcium oxide, strontium oxide, boronoxide, or calcium fluoride. Additional sintering aids include, but arenot limited to, alkali metal halides such as NaCl or KCl, and organiccompounds such as urea. In some embodiments, the assembly comprisesbetween about 0.01% and about 5%, between about 0.05% and about 5%,between about 0.1% and about 4%, or between about 0.3% and about 1% byweight of the flux material(s) or sintering aid(s). The sintering aidcan be intermixed with the raw materials. For example, in someembodiments, tetraethyl orthosilicate (TEOS) can be added to the rawmaterials to provide the desired amount of sintering aid. In oneembodiment, about 0.05% to about 5% by weight of TEOS is provided in theassembly. In some embodiments, the amount of TEOS may be between about0.3% and about 1% by weight.

Various plasticizers may also be included, in some embodiments, toreduce the glass transition temperature and/or improve flexibility ofthe ceramic. Non-limiting examples of plasticizers includedicarboxylic/tricarboxylic ester-based plasticizers, such asbis(2-ethylhexyl) phthalate, diisononyl phthalate,bis(n-butyl)phthalate, butyl benzyl phthalate, diisodecyl phthalate,di-n-octyl phthalate, diisooctyl phthalate, diethyl phthalate,diisobutyl phthalate, and di-n-hexyl phthalate; adipate-basedplasticizers, such as bis(2-ethylhexyl)adipate, dimethyl adipate,monomethyl adipate, and dioctyl adipate; sebacate-based plasticizers,such as dibutyl sebacate, and maleate; dibutyl maleate; diisobutylmaleate; polyalkylene glycols such as polyethylene glycol, polypropyleneglycol, and copolymers thereof; benzoates; epoxidized vegetable oils;sulfonamides, such as N-ethyl toluene sulfonamide,N-(2-hydroxypropyl)benzene sulfonamide, and N-(n-butyl)benzenesulfonamide; organophosphates, such as tricresyl phosphate, tributylphosphate; glycols/polyethers, such as triethylene glycol dihexanoate,tetraethylene glycol diheptanoate; alkyl citrates, such as triethylcitrate, acetyl triethyl citrate, tributyl citrate, acetyl tributylcitrate, trioctyl citrate, acetyl trioctyl citrate, trihexyl citrate,acetyl trihexyl citrate, butyryl trihexyl citrate, and trimethylcitrate; alkyl sulphonic acid phenyl ester; and mixtures thereof.

In some embodiments, the process may be made easier by occasionallyadding binder resin and solvent to the raw powders. A binder is anysubstance that improves adhesion of the particles of the compositionbeing heated to form a composite. Some non-limiting examples of bindersinclude polyvinyl alcohol, polyvinyl acetate, polyvinyl chloride,polyvinyl butyral, polystyrene, polyethylene glycol,polyvinylpyrrolidones, polyvinyl acetates, and polyvinyl butyrates, etc.In some, but not all, circumstances, it may be useful for the binder tobe sufficiently volatile that it can be completely removed or eliminatedfrom the precursor mixture during the sintering phase. Solvents whichmay be used include, but not limited to water, a lower alkanol such asbut not limited to denatured ethanol, methanol, isopropyl alcohol andmixtures thereof, preferably denatured ethanol, xylenes, cyclohexanone,acetone, toluene and methyl ethyl ketone, and mixtures thereof. In apreferred embodiment, the solvent is a mixture of xylenes and ethanol.

In some embodiments, the dispersants can be Flowen, fish oil, long chainpolymers, steric acid, oxidized Menhaden fish oil, dicarboxylic acidssuch succinic acid, orbitan monooleate, ethanedioic acid, propanedioicacid, pentanedioic acid, hexanedioic acid, heptanedioic acid,octanedioic acid, nonanedioic acid, decanedioic acid, o-phthalic acid,p-phthalic acid and mixtures thereof.

In some embodiments, organic particles can be added to impart porosityto one or more layers in the final emissive ceramic. The organicparticles can be any of those discussed above with respect to theassembly. For example, organic particles can be crosslinkedpolymethylmethacrylate beads. In some embodiments, the average largestdimension of the organic particles can be in the range of about 0.5 μmto about 50 μm. Moreover, the amount of the organic particles can beselected based on the desired pore volume percentage for any porousregion in the final emissive ceramic.

The mixture may then be subjected to comminution to form a slurry by,for example, ball milling the mixture for a time period in the range ofabout 0.5 hrs. to about 100 hrs. (preferably about 6 hrs. to about 48hrs., or more preferably about 12 hrs. to about 24 hrs.). The ballmilling may utilize milling balls that include materials other than thecomponents intermixed within the mixture (e.g., the milling balls may beZrO₂ for a mixture that forms YAG). In an embodiment, the ball millingincludes isolating the milling balls after a period of time byfiltration or other known methods of isolation. In some embodiments, theslurry has a viscosity in the range of about 10 cP to about 5000 cP(preferably about 100 cP to about 3000 cP, or more preferably about 400cP to 1000 cP).

Third, the slurry may be cast on a releasing substrate (e.g., a siliconecoated polyethylene teraphthalate substrate) to form a tape. Forexample, the slurry may be cast onto a moving carrier using a doctorblade and dried to form a tape. The thickness of the cast tape can beadjusted by changing the gap between the doctor blade and the movingcarrier. In some embodiments, the gap between the doctor blade and themoving carrier is in the range of about 0.125 mm to about 1.25 mm(preferably about 0.25 mm to about 1.00 mm, or more preferably about0.375 mm to about 0.75 mm). Meanwhile, the speed of the moving carriercan have a rate in the range of about 10 cm/min. to about 150 cm/min.(preferably about 30 cm/min. to about 100 cm/min., or more preferablyabout 40 cm/min. to about 60 cm/min.). By adjusting the moving carrierspeed and the gap between the doctor blade and moving carrier, the tapecan have a thickness between about 20 μm and about 300 μm. The tapes mayoptionally be cut into desired shapes after casting.

Two or more tapes are laminated to form the assembly. The laminationstep can include stacking two or more tapes (e.g., 2 to 100 tapes arestacked) and subjecting the stacked tapes to heat and uniaxial pressure(e.g., pressure perpendicular to the tape surface). For example, thestacked tapes may be heated above the glass transition temperature(T_(g)) of the binder contained in the tape and compressed uniaxiallyusing metal dies. In some embodiments, the uniaxial pressure is in therange of about 1 to about 500 MPa (preferably about 30 MPa to about 60MPa). In some embodiments, the heat and pressure is applied for a timeperiod in the range of about 1 min. to about 60 min. (preferably about15 min. to about 45 min., or more preferably about 30 min.). Thelamination step may optionally include forming various shapes (e.g.,holes or pillars) or patterns into the assembly by, for example, usingshaped dies.

Some embodiments of the assembly include at least one tape that containsyttrium aluminum garnet or its precursor, and is substantially free ofdopant. In addition, the assembly includes at least one tape havingyttrium aluminum garnet or its precursor, and a dopant. The stackedtapes can be arranged to obtain the desired assembly configuration andlayer thicknesses. For example, the tapes may be stacked to obtain theconfigurations illustrated in FIGS. 5A and 5B or 6A and 6B. Thethickness of the doped layer and one or more non-doped layers can bemodified by changing the number of tapes in the assembly. For example,to obtain a thicker non-doped layer, additional layers of tape can beadded to the assembly.

The assembly may optionally include at least one tape having organicparticles. In some embodiments, the assembly includes at least one tapehaving organic particles and at least one tape that is substantiallyfree of organic particles. In some embodiments, the assembly includes atleast one tape having a first size of organic particles and at least onetape having a second size of organic particles, where the first size oforganic particles is different than the second size of organicparticles. In some embodiments, the assembly includes at least one tapehaving a first amount by weight of organic particles and at least onetape having a second amount by weight of organic particles, where thefirst amount is different than the second amount.

Sintering the Assembly

The methods disclosed herein can include sintering the assembly toobtain the emissive ceramic. A person skilled in the art, guided by theteachings of the present application, can select appropriateconfigurations for the assembly and sintering conditions to obtain anemissive ceramic such as those disclosed herein having a dopantconcentration gradient.

Without being bound to any particular theory, it is believed that theprocesses disclosed herein cause dopant to diffuse out of the dopedlayer and into the non-doped layer. This diffusion process creates thedopant concentration gradient in the final emissive ceramic. Diffusionis generally described as the movement of a species (e.g., dopant) froma region of high concentration to a region of low concentration.Accordingly, dopant may diffuse from the doped layer having a highdopant concentration to the non-doped layer having a lower dopantconcentration.

The relative concentration across the thickness of the emissive ceramicobtained after sintering may be modeled using Fick's second law:

$\frac{\partial{C\left( {x,t} \right)}}{\partial t} = {D\frac{\partial^{2}{C\left( {x,t} \right)}}{\partial x^{2}}}$where x is a position along the thickness of the emissive ceramic; t istime; and C(x,t) is the dopant concentration at a position x. Varioussolutions are available to this equation depending upon the boundaryconditions (e.g., initial configuration of the assembly). See forexample J. Crank, The Mathematics of Diffusion, Oxford University Press,London 1956, which is hereby incorporated by reference in its entirety.

A simple solution is available for modeling an assembly configured asshown in FIGS. 5A and 5B. This solution, however, assumes that both thedoped layer and non-doped layer are sufficiently thick such that thedopant concentration remains unchanged near the first and secondsurfaces along the thickness. If this assumption is reasonable, thedopant concentration gradient could be modeled as

${{C\left( {x,t} \right)} = {\frac{1}{2}{C_{0}\left( {1 - {{erfc}\frac{x}{2\sqrt{Dt}}}} \right)}}},$where C₀ is the initial dopant concentration in the doped layer and x isthe distance apart from the interface between the two layers. Mostnotably, this equation shows the slope of the diffusion gradient nearthe interface decreases over time. This change is also proportional tothe diffusion coefficient. The diffusion coefficient generally increasesexponentially with temperature. Also, the half-maximum dopantconcentration will be near the interface between the two layers in thismodel.

Based on the above, it is apparent that the sintering conditions willaffect the final dopant concentration profile. For example, sinteringfor longer periods of time will reduce the slope of the concentrationgradient, and produce broader peaks. Meanwhile, increasing the sinteringtemperature will increase the diffusion coefficient, which canaccelerate diffusion and produce a reduced slope in a shorter timeperiod relative to lower sintering temperatures. Of course, theequations provided above merely provide a simplified model that may beused by the skilled artisan to select appropriate sintering conditionsto obtain a desired dopant concentration profile. The skilled artisanwill recognize that these equations merely serve as a guide because theunderlying assumptions are not always applicable. For example, Fick'ssecond law assumes that the diffusion coefficient is constant regardlessof the concentration. However, this assumption is not alwaysappropriate.

Accordingly, the processes disclosed herein include having at least aportion of the dopant diffuse out of the doped layer. In someembodiments, at least 30% of the dopant diffuses out of the doped layerduring the process. In some embodiments, at least 40% of the dopantdiffuses out of the doped layer during the process. In some embodiments,at least 50% of the dopant diffuses out of the doped layer during theprocess. In some embodiments, at least 60% of the dopant diffuses out ofthe doped layer during the process. In some embodiments, at least 70% ofthe dopant diffuses out of the doped layer during the process.

At least a portion of the dopant may diffuse into a non-doped layer. Forexample, about 30% of the dopant in the doped layer may diffuse into thenon-doped layer. In some embodiments, at least a portion of the dopantmay diffuse into a first non-doped layer and a second non-doped layer.As an example, at least about 20% of the dopant in the doped layer maydiffuse into the first non-doped layer, and at least about 20% of thedopant in the doped layer may diffuse into the second non-doped layer,

Some embodiments have a first amount of dopant in the doped layer thatdiffuses into a first non-doped layer and a second amount of dopant inthe doped layer that diffuses into a second non-doped layer duringsintering. The relative values of the first amount and second amount maybe modified, in part, by the geometry of the assembly. For example, athicker first non-doped layer may receive a higher amount dopantrelative to a thinner second non-doped layer under appropriateconditions. The ratio of the first amount and the second amount may, forexample, be in the range of about 4:1 and about 1:4; in the range ofabout 3:1 and about 1:3; or in the range of about 2:1 and 1:2. In someembodiments, the first amount and the second amount are about the same.

The first amount of dopant may, for example, be at least about 20% ofthe dopant in the doped layer; at least about 25% of the dopant in thedoped layer; at least about 30% of the dopant in the doped layer; atleast about 35% of the dopant in the doped layer; or at least about 40%of the dopant in the doped layer. The second amount of dopant may, forexample, be at least about 20% of the dopant in the doped layer; atleast about 25% of the dopant in the doped layer; at least about 30% ofthe dopant in the doped layer; at least about 35% of the dopant in thedoped layer; or at least about 40% of the dopant in the doped layer.

The sintering conditions may also be adjusted to control the maximumdopant concentration in the emissive ceramic relative to the initialmaximum dopant concentration of the doped layer in the assembly (e.g.,before sintering). For example, an assembly having a 0.5 at % Ce dopedlayer may be sintered to produce an emissive ceramic having a dopantconcentration gradient with a maximum dopant concentration of about 0.25at % Ce. Thus, in this case, the maximum dopant concentration is about50% of the initial maximum dopant concentration in the doped layer. Insome embodiments, the dopant concentration gradient of the emissiveceramic includes a maximum dopant concentration that is no more thanabout 65% of the initial dopant concentration in the doped layer of theassembly. The dopant concentration gradient of the emissive ceramic mayalso include, for example, a maximum dopant concentration that is nomore than about 60% of the initial dopant concentration in the dopedlayer of the assembly; no more than about 55% of the initial dopantconcentration in the doped layer of the assembly; no more than about 50%of the initial dopant concentration in the doped layer of the assembly;no more than about 40% of the initial dopant concentration in the dopedlayer of the assembly; or no more than about 25% of the initial dopantconcentration in the doped layer of the assembly.

Prior to sintering, an optional debinding process may be completed. Thedebinding process includes decomposing at least a portion of organiccomponents within the assembly (e.g., volatilize binders, organicparticles, and plasticizers within the assembly). As an example, theassembly may be heated in air to a temperature in the range of about300° C. to about 1200° C. (preferably about 500° C. to about 1000° C.,or more preferably about 800° C.) at a rate of about 0.1° C./min. toabout 10° C./min. (preferably about 0.3° C./min. to about 5° C./min., ormore preferably about 0.5° C./min. to about 1.5° C./min). The heatingstep may also include maintaining the temperature for a time period inthe range of about 30 min. to about 300 min., which may be selectedbased upon the thickness of the assembly.

The assembly may be sintered in a vacuum, air, O₂, H₂, H₂/N₂, or a noblegas (e.g., He, Ar, Kr, Xe, Rn, or combinations thereof) at a temperaturein the range of about 1200° C. to about 1900° C. (preferably about 1300°C. to about 1800° C., or more preferably about 1350° C. to about 1700°C.) for a time period in the range of about 1 hr. to about 20 hrs(preferably about 2 hrs. to about 10 hrs.). In some embodiments, thedebinding and sintering processes are completed in a single step.

The assembly may be sandwiched between cover plates during the heatingstep to reduce distortion (e.g., warping, cambering, bending, etc.) ofthe assembly. The cover plates may include materials having a meltingpoint above the temperatures applied during the heating step. Moreover,the cover plate may be sufficiently porous to permit transport ofvolatilized components through the covering plates. As an example, thecovering plate may be zirconium dioxide having a porosity of about 40%.

In some embodiments, the sintering or debindering conditions can beeffective to volatilize any organic particles in the assembly. This canyield porous region(s) in the assembly that correspond to the size,amount, and location of organic particles in the assembly. In someembodiments, the organic particles can be volatilized during a separateheating step.

Lighting Apparatus and Methods of Using the Emissive Ceramic

Some embodiments provide a lighting apparatus having a light source andan emissive ceramic configured to receive at least a portion of theradiation emitted by the light source. The emissive ceramic may includea yttrium aluminum garnet region with a dopant concentration gradient,such as any of those disclosed above.

The light source may, in some embodiments, be configured to emit blueradiation. The blue radiation may, for example, have a wavelength ofpeak emission between about 360 nm and about 500 nm. In someembodiments, the light source emits radiation having a wavelength ofpeak emission between about 450 nm and about 500 nm. Some embodimentsinclude a light source that is a semiconductor LED. As an example, thelight source may be an AlInGaN based single crystal semiconductormaterial coupled to an electric source.

FIG. 8 is an example of a lighting apparatus that may include theemissive ceramics disclosed herein. A submount 10 has a light source 15,such as a conventional base LED mounted thereon. The light source 15 isadjacent to emissive layer 30 which receives at least a portion of thelight emitted from the light source 15. An optional encapsulant resin 25is placed over the light source 15 and the emissive layer 30. Emissivelayer 30 can include any of the emissive ceramics disclosed in thepresent application.

In some embodiments, the lighting apparatus includes an emissive ceramic(e.g., emissive layer 30 depicted in FIG. 8) having a first porousregion and second porous region, where the first porous region has agreater porosity (e.g., higher average pore size and/or higher porevolume percentage) than the second porous region. The second porousregion may, for example, be disposed between the first porous region andthe light source (e.g., light source 15).

Also disclosed herein are methods of producing light that includeexposing any of the emissive ceramics disclosed in the presentapplication to a blue radiation. The blue radiation may, for example,have a wavelength of peak emission between about 360 nm and about 500nm. In some embodiments, the blue radiation has a wavelength of peakemission between about 450 nm and about 500 nm.

In some embodiments, the emissive ceramic (e.g., emissive layer 30depicted in FIG. 8) having a first porous region and second porousregion, where the first porous region has a greater porosity (e.g.,higher average pore size and/or higher pore volume percentage) than thesecond porous region. The second porous region may be disposed between afirst surface of the emissive ceramic and the first porous region. Themethod can include, for example, applying blue radiation to the firstsurface of the emissive ceramic.

EXAMPLES

Additional embodiments are disclosed in further detail in the followingexamples, which are not in any way intended to limit the scope of theclaims.

Example 1 Non-Emissive Layers for Laminated Composite (Undoped HostMaterial)

A 50 ml high purity Al₂O₃ ball mill jar was filled with 55 g ofY₂O₃-stabilized ZrO₂ balls having a 3 mm diameter. In a 20 ml glassvial, 0.153 g dispersant (Flowlen G-700. Kyoeisha), 2 ml xylene (FisherScientific, Laboratory grade) and 2 ml ethanol (Fisher Scientific,reagent alcohol) were mixed until the dispersant was dissolvedcompletely. The dispersant solution and tetraethoxysilane as sinteringaid (0.038 g, Fluka) were added to a ball mill jar.

Y₂O₃ powder (3.984 g, 99.99%, lot N-YT4CP, Nippon Yttrium Company Ltd.)with a BET surface area of 4.6 m²/g and Al₂O₃ powder (2.998 g, 99.99%,grade AKP-30, Sumitomo Chemicals Company Ltd.) with a BET surface areaof 6.6 m²/g were added to ball mill jar. The total powder weight was 7.0g and the ratio of Y₂O₃ to Al₂O₃ was at a stoichiometric ratio of 3:5. Afirst slurry was produced by mixing the Y₂O₃ powder, the Al₂O₃ powder,dispersant, tetraethoxysilane, xylenes, and ethanol by ball milling for24 hours.

A solution of binder and plasticizers was prepared by dissolving 3.5 gpoly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (Aldrich), 1.8 gbenzyl n-butyl phthalate (98%, Alfa Aesar), and 1.8 g polyethyleneglycol (Mn=400, Aldrich) in 12 ml xylene (Fisher Scientific, Laboratorygrade) and 12 ml ethanol (Fisher Scientific, reagent alcohol). A secondslurry was produced by adding 4 g of the binder solution into the firstslurry and then milling for another 24 hours. When ball milling wascomplete, the second slurry was passed through a syringe-aided metalscreen filter with pore size of 0.05 mm. Viscosity of second slurry wasadjusted to 400 centipoise (cP) by evaporating solvents in the slurrywhile stirring at room temperature. The slurry was then cast on areleasing substrate, e.g., silicone coated Mylar® carrier substrate(Tape Casting Warehouse) with an adjustable film applicator (Paul N.Gardner Company, Inc.) at a cast rate of 30 cm/min. The blade gap on thefilm applicator was set at 0.381 mm (15 mil). The cast tape was driedovernight at ambient atmosphere to produce a green sheet of about 100 μmthickness.

Example 2 Non-Emissive Layers for Laminated Composite (Al₂O₃ Material)

The non-emissive layers were made in accordance with Example 1, exceptthat Al₂O₃ (5 g, 99.99%, grade AKP-30, Sumitomo Chemicals Company Ltd.)with BET surface area of 6.6 m²/g was used instead of Y₂O₃ and Al₂O₃powder as described above for the Al₂O₃ green sheet preparation. A greensheet of about 100 um thickness was produced.

Example 3 Plasma Laminate for Emissive Layers

Plasma-produced amorphous yttrium aluminum oxide (with stoichiometryY:Al:O=3:5:12) powders (5.2 g) containing 1.0 at % cerium with respectto yttrium with a BET surface area of about 20 m²/g was added to a highpurity alumina combustion boat followed by annealing in a tube furnace(MTI GSL-1600) at heating ramp of 3-5° C./min to 1350° C. in air or 3%H₂/97% N₂ for 2 hrs. Then, it was cooled down to room temperature at aramp of 5° C./min. Yellow color powder with a BET surface area of 4.6m²/g was obtained after annealing.

A 50 ml high purity Al₂O₃ ball mill jar was filled with 24 g Y₂O₃stabilized ZrO₂ ball of 3 mm diameter. Then, in a 20 ml glass vial,0.084 g dispersant (Flowlen G-700. Kyoeisha), 2 ml xylene (FisherScientific, Laboratory grade), and 2 ml ethanol (Fisher Scientific,reagent alcohol) were mixed until the dispersant was dissolvedcompletely. The dispersant solution and tetraethoxysilane as a sinteringaid (0.045 g 99.0% pure, Fluka) were added to a ball mill jar. Theannealed plasma YAG powder (3.0 g) with a BET surface area of 4.6 m²/g,was added to a ball mill jar. The first slurry was produced by mixingthe YAG powder, dispersant, tetraethoxysilane, xylenes, and ethanol byball milling for 24 hours.

A solution of binder and plasticizers was prepared by dissolving 5.25 gpoly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (Aldrich), 2.6 gbenzyl n-butyl phthalate (98%, Alfa Aesar), and 2.6 g polyethyleneglycol (Mn=400, Aldrich) in 18 ml xylene (Fisher Scientific, Laboratorygrade) and 18 ml ethanol (Fisher Scientific, reagent alcohol). A secondslurry was produced by adding 1.2 g of the binder solution into thefirst slurry and then milling another 24 hours. When ball milling wascomplete, the second slurry was passed through a syringe-aided metalscreen filter with pore size of 0.05 mm. The viscosity of the secondslurry was adjusted to 400 centipoise (cP) by evaporating solvents inthe slurry while being stirred at room temperature. The slurry was thencast on a releasing substrate, e.g., silicone coated Mylar® carriersubstrate (Tape Casting Warehouse) with an adjustable film applicator(Paul N. Gardner Company, Inc.) at a cast rate of 30 cm/min. The bladegap on the film applicator was set at 0.127 mm (5-mil). The cast tapewas dried overnight at ambient atmosphere to produce a yellow-coloredgreen sheet of about 40 μm thickness.

Example 4 YAG Ceramics for Setting Up Calibration Curve in TOF-SIMS(Time-of-Flight Secondary Ion Mass Spectroscopy)

Plasma-YAG green sheet for making YAG:Ce ceramics as standard samples toset up a calibration curve were prepared by following the sameprocessing procedures as described in Example 3. The prepared greensheets contained 0.5 at %, 1.0 at % and 1.5 at % of Ce respectively. AYAG green sheet was prepared by a solid state reaction using the sameprocessing procedures as described in Example 1. The prepared greensheet did not contain Ce.

Example 5 Laminate Composites

The green sheets of non-emissive (e.g., un-doped host material) compriseY₂O₃ and Al₂O₃ powders at an atomic ratio of 3:5, each sheet having athickness of about 100 μm. The emissive green sheets comprisepre-annealed plasma-synthesized YAG:Ce powder containing about 1.0 at %Ce (40 μm thickness). The Al₂O₃ green sheets comprise Al₂O₃ powders witha thickness of 100 μm. These green sheets were cut into circular shapesof about 13 mm in diameter with laser cutter. By varying the number ofcut non-emissive, emissive and Al₂O₃ pieces, several laminate compositeswere constituted as set forth in Table 1.

TABLE 1 Assembly Configurations Sample Doped layer Non-doped layer 1[2-1-2] 1 × (40 μm) 2 (2 × (100 μm) (Y_(0.99)Ce_(0.01))₃Al₅O₁₂ YAGnon-doped) 2 [2-3-2] 1 × (40 μm) 2 (2 × (100 μm)(Y_(0.99)5Ce_(0.005))₃Al₅O₁₂ YAG non-doped) 1 × (40 μm)(Y_(0.99)Ce_(0.01))₃Al₅O₁₂ 1 × (40 μm) (Y_(0.995)Ce_(0.005))₃Al₅O₁₂ 3[2-3-2] 3 × (40 μm) 2 (2 × (100 μm) (Y_(0.99)Ce_(0.01))₃Al₅O₁₂ Al₂O₃)

In Table 1, [2-1-2] refers to a lamination of two non-emissive layers oneach side of one emissive layer. Meanwhile, [2-3-2] refers to alamination of two non-emissive layers on each side of a composite 3×40μm emissive layer.

The respective pieces of punched circular-shaped undoped green sheet andplasma YAG:Ce green sheet were placed between circular dies withmirror-polished surfaces and heated on a hot plate to 80° C., followedby compression in a hydraulic press at a uniaxial pressure of 5 metrictons and held at that pressure for 5 minutes. Laminated composites ofemissive and non-emissive layers were thus produced.

For debindering, laminated green sheets were sandwiched between ZrO₂cover plates (1 mm in thickness, grade 42510-X, ESL Electroscience Inc.)and placed on an Al₂O₃ plate of 5 mm thick; then heated in a tubefurnace in air at a ramp rate of 0.5° C./min to 800° C. and held for 2hours to remove the organic components from the green sheets.

After debindering, the assembly was annealed at 1500° C. at 20 Torr for5 hours at a heating rate of 1° C./min to complete conversion fromnon-garnet phases of Y—Al—O in the non-emissive layer, including, butnot limited to, amorphous yttrium oxides, YAP, YAM or Y₂O₃ and Al₂O₃ toyttrium aluminum garnet (YAG) phase and increase the final YAG grainsize.

Following the first annealing, the assembly were further sintered in avacuum of 10⁻³ Torr at about 1700° C. for 5 hours at a heating rate of5° C./min and a cooling rate of 10° C./min to room temperature toproduce a translucent YAG ceramic sheet of about 0.4 mm thickness.Brownish sintered ceramic sheets were reoxidized in a furnace at 20 Torrat 1400° C. for 2 hrs at heating and cooling rates of 10° C./min and 20°C./min 20 Torr respectively. The sintered laminated composite exhibitedtransmittance greater than 70% at 800 nm. When irradiated with a blueLED with peak emission wavelength at 455 nm, no clear boundary betweenemissive and non-emissive layer can be observed, which indicates thatsignificant diffusion of cerium occurred from the doped plasma YAG layerto the non-doped YAG layer.

Example 6 IQE Measurement Results of Laminated Composite

IQE measurements were performed with an Otsuka Electronics MCPD 7000multi channel photo detector system (Osaka, JPN) together with requiredoptical components such as integrating spheres, light sources,monochromator, optical fibers, and sample holder as described below.

The YAG:Ce phosphor ceramics plate constructed as described above, witha diameter of about 11 mm, were placed on a light emitting diode (LED)with peak wavelength at 455 nm with acrylic lens which had a refractiveindex of about 1.45. An LED with YAG:Ce was set up inside integrationsphere. The YAG:Ce ceramics plate was irradiated by the LED and theoptical radiation of blue LED and YAG:Ce ceramics were recordedrespectively. Next, the YAG:Ce ceramics plate was removed from LED, andthen the radiation of blue LED with the acrylic lens were measured.

IQE was calculated by integration of the radiation difference from theblue only LED and blue LED/YAG:Ce combination as shown in FIG. 9. TheIQE of the laminated composite samples was determined to be 90%(plasma-generated YAG with 40 μm thick layer of YAG:Ce (1.0 at % Ce)between 200 μm thick YAG undoped layers), 85% (plasma-generated YAG with120 μm thick layer of YAG:Ce (1% wt Ce) between 200 um thick YAG undopedlayers), and 55% (plasma-generated YAG with 40 μm thick layer of YAG:Ce(1% wt Ce) between 200 μm thick Al₂O₃ undoped layers.

Example 7 TOF-SIMS Analysis

The sintered ceramic obtained from Sample 1 in Example 5 was analyzed byTOF-SIMS (Time-Of-Flight Secondary Ion Mass Spectroscopy) and theresults are shown in FIG. 10. As can be seen, Ce diffused into thenon-doped layer as indicated by the tailing amount of Ce extending fromabout point A (the interface between the emissive and non-emissivelayers) into the non-emissive layer at least about 100 μm. The Ceconcentration decreased to about 0.45 units.

The sintered ceramic obtained from Sample 2 in Example 5 was alsoanalyzed by TOF-SIMS. As indicated in FIG. 11, the use of 120 μm/YAG (0%Ce) in conjunction with 200 μmm layers of YAG undoped layer reduced thediffusion of Ce. This produced a maximum concentration that remainedessentially unchanged (e.g., about 10.0 units) after sintering.

As a comparison, the sintered ceramic obtained from Sample 3 in Example5 was also analyzed was also analyzed by TOF-SIMS. As indicated in FIG.12, the use of Al₂O₃ layer substantially blocked the diffusion of Ce andresulted in a non-emissive layer substantially free of dopant. Also, themaximum dopant concentration remained substantially the same (e.g.,about 10.0 units) after sintering.

Example 8

Sixteen different assemblies were prepared and sintered according toExamples 1, 3, and 5. The assembly included a doped layer sandwichedbetween two non-doped layers of about 200 μm thick (2 times 100 μmsheets laminated on each side). The doped layer had thicknesses rangingfrom about 30 to about 160 μm and concentrations ranging from 0.2% toabout 1.25% Ce. The IQE for each composite tablet was determined as setforth above in Example 5. The results are shown in FIG. 13.

Example 9

Nine different assemblies were prepared and sintered according toExamples 1, 3, and 5. The assembly included a doped layer sandwichedbetween two non-doped layers of about 200 μm thick (2 times 100 μmsheets laminated on each side). The doped layer had thicknesses rangingfrom about 20 to about 50 μm and concentrations ranging from 1.25% toabout 2.0% Ce. The IQE for each composite tablet was determined as setforth above in Example 5. The results are shown in FIG. 14. FIG. 15depicts the IQE plotted as a function of Ce concentration. The Figureindicates that the increasing IQE appears to plateau as theconcentration exceeds 2.0 at %.

Example 10

The green sheets of non-emissive (e.g., un-doped) comprise Y₂O₃ andAl₂O₃ powders at an atomic ratio of 3:5, each sheet having a thicknessof about 100 μm. The emissive green sheets comprise pre-annealedplasma-synthesized YAG:Ce powder containing about 2.0 at % Ce (37 μmthickness). A laminate of green sheet was constructed by placing twogreen sheets of non-emissive (200 μm) at each side of emissive layer.These green sheets were cut into circular shapes of about 13 mm indiameter with laser cutter. In a comparison sample, YAG:Ce laminateswere constructed with a emissive layer comprising plasma-YAG:Ce greensheet containing 1.0 at % of Ce with plasma-YAG green sheets containing0.5 at % of Ce on each side as showed in FIG. 16. The Ce concentrationin the doped layer is roughly same, while the Ce concentration gradientin each structure is different. By following the procedures as describedabove for annealing and evaluation, IQE values were obtained and shownin FIG. 17. Sample labeled as 050 showed IQE of 0.93, whereas the samplelabeled as 051 showed IQE of 0.71. This difference shows the dopantconcentration gradient affects the IQE. A thin emissive layer withhigher Ce concentration appears to be favorable for achieving high IQEthrough diffusion without concentration quenching.

Example 11

The green sheets of non-emissive (e.g., un-doped) comprise Y₂O₃ andAl₂O₃ powders at an atomic ratio of 3:5, each sheet having a thicknessof about 100 μm were prepared as set forth in Example 1. In Samples5-10, the emissive green sheets comprise pre-annealed plasma-synthesizedYAG:Ce powder containing about 0.5 at % Ce (37 μm thickness), 1.5 at %Ce (28 μm thickness), 2.0 at % Ce (23 μm thickness), 0.2 at % Ce (160 μmthickness), 1.25 at % Ce (100 μm thickness), and 4.0 at % Ce (16 μmthickness) were prepared as set forth in Example 3, except that therespective desired Ce amount was used to attain the desired at % and therespective blade gap was selected to attain the desired preen sheetthickness, e.g., 0.127 mm (5-mil) for 37 μm, 0.127 mm (5-mil) for 28 μm,0.127 mm (5-mil) for 23 μm, 0.127 mm (5-mil) for 40 μm and 4×40 umstacking for 160 μm, 0.254 mm (10 mil) for 50 μm and 2×50 μm stackingfor 100 μm, 0.1016 mm (4-mil) for 16 μm.

In Sample 11, the emissive green sheet containing about 1.0 at % Ce (50μm thickness) was prepared as set forth in Example 3 except the Ceamount was used to attain the desired at % and the respective blade gapwas selected to attain the desired green sheet thickness, e.g., 0.254 mm(10 mil) for 50 μm, and comprised Y₂O₃ and Al₂O₃ powders as described inExample 1 (Y₂O₃ powder [3.984 g, 99.99%, lot N-YT4CP, Nippon YttriumCompany Ltd.] with a BET surface area of 4.6 m²/g and Al₂O₃ powder[2.998 g, 99.99%, grade AKP-30, Sumitomo Chemicals Company Ltd.] with aBET surface area of 6.6 m²/g).

A laminate of green sheet was constructed by placing two green sheets ofnon-emissive (200 um) at each side of the emissive layer as shown inFIG. 6. These green sheets were cut into circular shapes of about 13 mmin diameter using a laser cutter. By following the procedures asdescribed in Example 4 for annealing except that they were annealed at800° C. in air for removal of organic constituent and sintered at 1700°C. for about 5 hours in vacuum and evaluation, IQE values were obtainedas shown in TABLE 2.

TABLE 2 Assembly Configurations and Resulting IQE for the EmissiveCeramic Non-doped Layer Doped Layer Ce Conc. in Sample thicknessesThickness Doped Layer IQE  5 200 μm  40 μm  0.5 at % 0.93  6 200 μm  28μm  1.5 at % 0.90  7 200 μm  23 μm  2.0 at % 0.95  8 200 μm 160 μm  0.2at % 0.67  9 200 μm 100 μm 1.25 at % 0.86 10 200 μm  16 μm  4.0 at %0.79 11 200 μm  50 μm  1.0 at % 0.94

Example 12

Samples 5-10 as prepared in Example 11 were also analyzed to provide aTime of Flight secondary Ion Mass Spectroscopy profile.

YAG:Ce ceramic plates were embedded in epoxy resin. After curing theembedding epoxy, the ceramic samples were mechanically polished. Surfacecontamination of the samples was removed by using Bismuth ionsputtering.

TOF-SIMS analysis was performed using a TOF-SIMS 5 (ION-TOF GmbH,Munster, Germany). The surface of the ceramic samples was irradiatedwith pulsing bismuth primary ions (Bi+) accelerated at 25 kV. To obtainmass spectrum, positive secondary ions emitted from samples werecollected by reflectron-type time-of-flight mass analyzer and detectedby a micro-channel plate detector with a post-acceleration energy of 10kV. A low-energy electron flood gun was utilized for chargeneutralization in the analysis mode.

Ion images (Al+, Ce+, Y+ etc.) were first prepared. Then, profile curvesof each ion along the thickness direction of the ceramics werereconstructed from the ion images. A calibration curve, which representsa relation between signal intensity and the ion concentration wascreated. Establishment of the calibration curve, as shown in FIG. 18,provided by plotting the signal intensity of the TOF-SIMS instrumentwith a plurality of standard samples of known ion concentration. Theactual atomic percent of Ce+ ion along the thickness direction of eachYAG:Ce ceramic plate was eventually analyzed with this calibrationcurve.

FIGS. 19-24 show the respective TOF-SIMS profiles of Samples 5-10 inExample 11. The Ce+ TOF-SIMS profiles of Sample 8 and Sample 9 aredistinctly different from those of Samples 5-7.

Example 13 Doped Layer with Polymer Beads

A 50 ml high purity Al₂O₃ ball mill jar was filled with 55 g ofY₂O₃-stabilized ZrO₂ balls having a 3 mm diameter. In a 20 ml glassvial, 0.153 g dispersant (Flowlen G-700. Kyoeisha), 2 ml xylene (FisherScientific, Laboratory grade) and 2 ml ethanol (Fisher Scientific,reagent alcohol) were mixed until the dispersant was dissolvedcompletely. The dispersant solution and tetraethoxysilane as sinteringaid (0.038 g, Fluka) were added to a ball mill jar.

Y₂O₃ powder (3.94 g, 99.99%, lot N-YT4CP, Nippon Yttrium Company Ltd.)with a BET surface area of 4.6 m²/g and Al₂O₃ powder (3.00 g, 99.99%,grade AKP-30, Sumitomo Chemicals Company Ltd.) with a BET surface areaof 6.6 m²/g and Ce(NO₃)₃6H₂O (0.153 g, 99.99%,Aldrich) were added toball mill jar. The total powder weight was 7.0 g and the ratio of Y₂O₃to Al₂O₃ was at a stoichiometric ratio of 3:5. A first slurry wasproduced by mixing the Y₂O₃ powder, the Al₂O₃ powder, the Ce(NO₃)₃6H₂O,dispersant, tetraethoxysilane, xylenes, and ethanol by ball milling for24 hours.

A solution of binder and plasticizers was prepared by dissolving 3.5 gpoly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (Aldrich), 1.8 gbenzyl n-butyl phthalate (98%, Alfa Aesar), and 1.8 g polyethyleneglycol (Mn=400, Aldrich) in 12 ml xylene (Fisher Scientific, Laboratorygrade) and 12 ml ethanol (Fisher Scientific, reagent alcohol). A secondslurry was produced by adding 4 g of the binder solution into the firstslurry and then milling for another 24 hours. When ball milling wascomplete, the second slurry was passed through a syringe-aided metalscreen filter with pore size of 0.05 mm.

A 50 ml high purity Al₂O₃ ball mill jar was filled with 15 g ofY₂O₃-stabilized ZrO₂ balls having a 3 mm diameter. 10 g second slurryand 0.081 g polymeric beads (1 μm, Cross-linked-polymethylmethacrylateresin, Nippon Shokubai, Epostar MA1001) were added into the Al₂O₃ ballmill jar and then milling for 4 hrs. Viscosity of the slurry wasadjusted to 400 centipoise (cP) by evaporating solvents in the slurrywhile stirring at room temperature. The slurry was then cast on areleasing substrate, e.g., silicone coated Mylar® carrier substrate(Tape Casting Warehouse) with an adjustable film applicator (Paul N.Gardner Company, Inc.) at a cast rate of 30 cm/min. The blade gap on thefilm applicator was set at 0.127 mm (5 mil). The cast tape was driedovernight at ambient atmosphere to produce a green sheet of about 50 μmthickness.

Example 14 Non-Doped Layer with Polymer Beads

A 50 ml high purity Al₂O₃ ball mill jar was filled with 55 g ofY₂O₃-stabilized ZrO₂ balls having a 3 mm diameter. In a 20 ml glassvial, 0.153 g dispersant (Flowlen G-700. Kyoeisha), 2 ml xylene (FisherScientific, Laboratory grade) and 2 ml ethanol (Fisher Scientific,reagent alcohol) were mixed until the dispersant was dissolvedcompletely. The dispersant solution and tetraethoxysilane as sinteringaid (0.038 g, Fluka) were added to a ball mill jar.

Y₂O₃ powder (3.94 g, 99.99%, lot N-YT4CP, Nippon Yttrium Company Ltd.)with a BET surface area of 4.6 m²/g and Al₂O₃ powder (3.00 g, 99.99%,grade AKP-30, Sumitomo Chemicals Company Ltd.) with a BET surface areaof 6.6 m²/g were added to ball mill jar. The total powder weight was 7.0g and the ratio of Y₂O₃ to Al₂O₃ was at a stoichiometric ratio of 3:5. Afirst slurry was produced by mixing the Y₂O₃ powder, the Al₂O₃ powder,tetraethoxysilane, xylenes, and ethanol by ball milling for 24 hours.

A solution of binder and plasticizers was prepared by dissolving 3.5 gpoly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (Aldrich), 1.8 gbenzyl n-butyl phthalate (98%, Alfa Aesar), and 1.8 g polyethyleneglycol (Mn=400, Aldrich) in 12 ml xylene (Fisher Scientific, Laboratorygrade) and 12 ml ethanol (Fisher Scientific, reagent alcohol). A secondslurry was produced by adding 4 g of the binder solution into the firstslurry and then milling for another 24 hours. When ball milling wascomplete, the second slurry was passed through a syringe-aided metalscreen filter with pore size of 0.05 mm.

A 50 ml high purity Al₂O₃ ball mill jar was filled with 15 g ofY₂O₃-stabilized ZrO₂ balls having a 3 mm diameter. 10 g second slurryand 2.284 g polymeric beads (4 μm, Cross-linked-polymethylmethacrylateresin, Nippon Shokubai, Epostar MA1004) were added into the Al₂O₃ ballmill jar and then milling for 4 hrs. Viscosity of the slurry wasadjusted to 400 centipoise (cP) by evaporating solvents in the slurrywhile stirring at room temperature. The slurry was then cast on areleasing substrate, e.g., silicone coated Mylar® carrier substrate(Tape Casting Warehouse) with an adjustable film applicator (Paul N.Gardner Company, Inc.) at a cast rate of 30 cm/min. The blade gap on thefilm applicator was set at 0.101 mm (4 mil). The cast tape was driedovernight at ambient atmosphere to produce a green sheet of about 40 μmthickness.

Example 15 Emissive Ceramics with Porous Layers

Two kinds of non-doped layers (e.g., un-doped host material) includingY₂O₃ and Al₂O₃ powders at an atomic ratio of 3:5, one with a thicknessof 100 μm and without polymer beads, and another having a thickness ofbout 40 μm with 60 vol % of polymer beads of 4 μm, which were insertedbetween doped layers and non-doped layers containing no polymer beadsfor the purpose of improving angular dependence of chromaticity byscattering of photoluminescence. The doped layers include Y₂O₃ and Al₂O₃containing about 1.0 at % Ce (50 μm thickness) and 5 vol % polymer beadswith size of 1 μm to reduce the outcoupling loss of photoluminescence bywaveguide effect. These layers were cut into circular shapes of about 13mm in diameter with laser cutter. A symmetrical configuration oflaminate composite was constituted as set forth in Table 3.

TABLE 3 Assembly Configurations Beads Beads loading size Thickness Layer(vol %) (μm) (μm) Composition 1 Non-emissive  0 — 100 Y₃Al₅O₁₂ 2Non-emissive 60 4  40 Y₃Al₅O₁₂ 3 Emissive  5 1  50(Y_(0.99)Ce_(0.01))₃Al₅O₁₂ 4 Non-emissive 60 4  40 Y₃Al₅O₁₂ 5 Nonemissive  0 — 100 Y₃Al₅O₁₂

The respective pieces of punched circular-shaped undoped green sheet anddoped green sheet were placed between circular dies with mirror-polishedsurfaces and heated on a hot plate to 80° C., followed by compression ina hydraulic press at a uniaxial pressure of 5 metric tons and held atthat pressure for 5 minutes. Laminated composites of doped and non-dopedlayers were thus produced.

For debindering, the assembly was sandwiched between ZrO₂ cover plates(1 mm in thickness, grade 42510-X, ESL Electroscience Inc.) and placedon an Al₂O₃ plate of 5 mm thick; then heated in a tube furnace in air ata ramp rate of 0.5° C./min to 800° C. and held for 2 hours to remove theorganic components from the green sheets. In this process, porousstructure formed in part of layer containing polymer beads by burn-outof polymer components.

After debindering, the assembly was annealed at 1500° C. at 20 Torr for5 hours at a heating rate of 1° C./min to complete conversion fromnon-garnet phases of Y—Al—O in the non-emissive layer, including, butnot limited to, amorphous yttrium oxides, YAP, YAM or Y₂O₃ and Al₂O₃ toyttrium aluminum garnet (YAG) phase and increase the final YAG grainsize.

Following the first annealing, the assembly were further sintered in avacuum of 10⁻³ Torr at about 1700° C. for 5 hours at a heating rate of5° C./min and a cooling rate of 10° C./min to room temperature toproduce a translucent YAG ceramic sheet of about 0.4 mm thickness.Brownish sintered ceramic sheets were reoxidized in a furnace at 20 Torrat 1400° C. for 2 hrs at heating and cooling rates of 10° C./min and 20°C./min 20 Torr respectively. The sintered laminated composite exhibitedtransmittance greater than 50% at 800 nm. When irradiated with a blueLED with peak emission wavelength at 455 nm, no clear boundary betweenemissive and non-emissive layer can be observed, which indicates thatsignificant diffusion of cerium occurred from the doped plasma YAG layerto the non-doped YAG layer.

Example 16 IQE and Angular Dependence Measurement Results of LaminateComposite

IQE measurement was performed using the equipment and setup as describedin EXAMPLE 6. The IQE of the laminated composite sample producedaccording to Example 15 having the porous layers was 90%.

A setup as shown in FIG. 26 was used for measuring angular dependence ofchromaticity coordinate for the YAG phosphor ceramics. The setupcomprises a high sensitivity multichannel photodetector and anintegrating sphere (OTSUKA ELECTRONICS, Osaka Japan) for collecting thephotoluminescence signal. A 360 degree rotation platform and translationstages allowed the variation of angle of photoluminescence source withrespect to the integrating sphere in the range of 0 to 360 degree.Phosphor ceramics diced into 1.0 mm by 1.0 mm was attached to blue LEDsource (Cree Inc.) by silicone resin and cured at 150° C. for 1 hr inambient atmosphere to form a LED module. The LED module was mounted onto a sample holder attached to the rotation platform. To measure theangular dependence, DC voltage of 3.0 V and current of 0.1 A was appliedto the LED module. Starting at 0 degree defined as the position with YAGceramics surface facing the normal to integrating sphere,photoluminescence of YAG ceramics was recorded at 10 degree intervalclockwise and counter-clockwise to 90 degree, meaning the edge of YAGceramics directed to the integrating sphere.

Angular dependence of YAG ceramics was obtained by plotting thechromaticity coordinate against angle as shown in FIG. 27. The squaredata points were measured for the emissive ceramic prepared according toExample 15, while the circle data points were measured for the emissiveceramic prepared according to Example 11 (i.e., without any porousregions). As is clear from FIG. 27, including the porous regions in theemissive ceramic improves the isotropy of the light emitting properties.

What is claimed is:
 1. An emissive ceramic comprising a yttrium aluminumgarnet (YAG) region and a dopant having a concentration gradient along athickness of the YAG region between a first surface and a secondsurface, wherein said concentration gradient comprises a maximum dopantconcentration, a first half-maximum dopant concentration, and a firstslope at or near the first half-maximum dopant concentration, wherein anabsolute value of the first slope is in the range of about 0.001 andabout 0.004 (at %/μm).
 2. The emissive ceramic of claim 1, wherein themaximum dopant concentration is in the range of about 0.25 at % to about0.5 at %.
 3. The emissive ceramic of claim 1, wherein the maximum dopantconcentration is located no more than about 100 μm away from the firstor second surface.
 4. The emissive ceramic of claim 1, wherein themaximum dopant concentration is located no more than about 100 μm awayfrom the center of the thickness of the YAG region.
 5. The emissiveceramic of claim 1, wherein the first half-maximum dopant concentrationis located at least about 25 μm away from the location of the maximumdopant concentration.
 6. The emissive ceramic of claim 1, wherein thefirst half-maximum dopant concentration is located at least 50 μm awayfrom the first and second surfaces.
 7. The emissive ceramic of claim 1,further comprising a second half-maximum dopant concentration located atleast 25 μm away from the location of the first half-maximum dopantconcentration.
 8. The emissive ceramic of claim 7, further comprising asecond slope at or near the second half-maximum dopant concentration,wherein an absolute value of the second slope is in the range of 0.001and 0.004 (at %/μm).
 9. The emissive ceramic of claim 8, wherein theabsolute value of the first slope is about the same as the absolutevalue of the second slope.
 10. The emissive ceramic of claim 1, whereinthe dopant concentration gradient comprises a peak having a full-widthat half-maximum in the range of about 50 μm to about 400 μm.
 11. Theemissive ceramic of claim 1 wherein the YAG region further comprises afirst porous region.
 12. The emissive ceramic of claim 11, wherein thefirst porous region has a pore volume in the range of about 0.5% toabout 80%.
 13. The emissive ceramic of claim 11, wherein the firstporous region comprises pores having an average size in the range ofabout 0.5 μm to about 50 μm.
 14. The emissive ceramic of claim 11,wherein the YAG region further comprises a first non-porous region and asecond non-porous region, and wherein the first porous region isdisposed between the first non-porous region and the second non-porousregion.
 15. The emissive ceramic of claim 11, wherein the YAG regionfurther comprises a first non-porous region and a second porous region,and wherein the first non-porous region is disposed between the firstporous region and the second porous region.
 16. The emissive ceramic ofclaim 11, wherein the first porous region has a thickness in the rangeof about 10 μm to about 400 μm.
 17. An emissive ceramic comprising ayttrium aluminum garnet (YAG) region and a dopant having a concentrationgradient along a thickness of the YAG region, wherein said concentrationgradient comprises a maximum dopant concentration, a first half-maximumdopant concentration, and a first slope at or near the firsthalf-maximum dopant concentration, wherein an absolute value of thefirst slope is in the range of about one-eighth of the maximum dopantconcentration divided by the thickness and about two times the maximumdopant concentration divided by the thickness.
 18. The emissive ceramicof claim 17, wherein the maximum dopant concentration is located betweenabout one-quarter and about three-quarters along the thickness of theYAG region.
 19. The emissive ceramic of claim 17, wherein the emissiveceramic exhibits an internal quantum efficiency (IQE) of at least about0.80 when exposed to radiation having a wavelength of about 455 nm.